@Bazim if you are not familiar, download the free app Mediainfo. Start the program, select the 'View' tab, scroll down and select 'Text' mode. Then select the 'File' tab and navigate to your source media files and load one of them. Then post a screenshot of that report. The report from Mediainfo provides a bunch of information that will help members understand what you have and make a better suggestion.

In addition also post screenshots of your Project settings, and as @j-v request, a screenshot of your Preview window and the area around it, so members can determine how your Preview quality is setup. All of this will help others help you.

Also if you can post the specs of your computer, CPU, GPU, memory, drives, etc.

Sourcefiles : 1080p@60
Projectsettings -> source file
Settings for my preview screen? 10 GB ram usage

My PC:
i7-5820k
32 GB DDR4 RAM
GTX1080

@Bazim Are your source files actually 60p or 59.940p? Are they variable frame rate, or constant frame rate? What codec was used to encode? All these things are reported by the Mediainfo app I suggested above, and may be important for members to help.

Are you saying your Project settings are matched to your source?

Preview Quality is displayed above the Preview window, it could be Draft, Good, Preview, Best and each of those have an option for Auto, Full, Half, and Quarter, which is yours set for?

How are you zooming in on the timeline? Is this is reference to using Pan & Crop or Track Motion? If not, what is it in reference to?

Constant Frame Rate. Preview settings doesn't matter. It lagging on every setting. Zooming with mouse wheel. When i make a proxy video the preview stop lagging, but when i add text and i get to the point of this text. Preview window starts to lag and starts to be fine after the text.

@Bazim Thank You for the information, but we are still information to be of any help. Can you post a screenshot of your source media using Mediainfo in text mode, a screenshot of your project properties, a screenshot of your Movie Studio full screen display. And what are you referring to above "10 GB ram usage"?

The behavior you describe is pretty typical of being on the edge of your CPU capabilities. Some systems just will not playback at full speed when encountering things on a track such as FX's, transitions, etc. For that you can use Dynamic Ram Preview or Selective PreRender (this being the better I think), to view areas on a track/timeline that lag in preview.

General
Complete name                            : D:\Videa\OBS\my_best_game.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/avc1/mp41)
File size                                : 76.8 GiB
Duration                                 : 1 h 55 min
Overall bit rate mode                    : Variable
Overall bit rate                         : 95.1 Mb/s
Writing application                      : Lavf57.84.100

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.2
Format settings                          : CABAC / 2 Ref Frames
Format settings, CABAC                   : Yes
Format settings, RefFrames               : 2 frames
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 1 h 55 min
Bit rate mode                            : Variable
Bit rate                                 : 95.0 Mb/s
Maximum bit rate                         : 40.0 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 60.000 FPS
Minimum frame rate                       : 58.824 FPS
Maximum frame rate                       : 62.500 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.763
Stream size                              : 76.6 GiB (100%)
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.470 System B/G
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 1 h 55 min
Bit rate mode                            : Constant
Bit rate                                 : 129 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 44.1 kHz
Frame rate                               : 43.066 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 106 MiB (0%)
Default                                  : Yes
Alternate group                          : 1

10GB preview i set this in options

Display aspect ratio                     : 16:9
Frame rate mode                          : Variable
Frame rate                               : 60.000 FPS
Minimum frame rate                       : 58.824 FPS
Maximum frame rate                       : 62.500 FPS

I think this is giving you the troubles
Vegas does not like variable framerates.
Make your screenplay with steady contant framerate or use OBS, that one can give video where Vegas has no troubles with.
This file you can better let go trough a videoconverter who makes constant framerate.

I'm using OBS.

@Bazim you stated that your source was constant frame rate, but according to the source media specs you have posted (Thank You), it is variable frame rate. I don't use OBS, but I think it can be configured to record constant frame rate. You could use a program like Avidemux or Handbrake to convert the source media you already have recorded from VFR to CFR and see if that helps with you playback speed. I would start a fresh project to perform the test.

"10GB preview i set this in options", I assume by this you have changed the '200' Mb default in Preferences > Video tab 'Dynamic RAM preview max (MB):' to 10 Gb, if that is correct, increasing this for normal preview is not going to help. You can use an increase in this option that may help when you are doing Dynamic RAM previews, Tools > Build Dynamic RAM Preview (Ctrl+B), otherwise you are probably better off to return this to the default of 200 Mb.

Can you post the requested screenshot of your project properties, a screenshot of your Movie Studio full screen display?

Don't see constant frame rate option in OBS so i thought it was a default one.

http://www.postimg.cz/i/x926

And when I added a CFR video still when it go to the added text or transition preview video starts lagging.

EDIT //
Example : https://sendvid.com/hgvlz2zf

@Bazim Thank You for posting the Project settings. I'm not familiar with OBS and I know it changes things around through various version, so maybe it does have CFR, although maybe the option is listed as VFR with a check box.

I would recommend downloading the free app Avidemux, converting one of your source media files into constant frame rate and see how that previews for you.

Look at that video link. That is a constant frame rate video, but still preview lags when it hits the added text.

BTW.: In the OBS forum i found this : For some reason the media info for OBS MP4s (that I've seen anyways) is always variable, even if you recorded at constant. Its just metadata though, doesn't mean it really is variable. Have you tried the files anyways to see if they work?

And thats true. If i use medainfo of original FLV file it says CFR, but after remuxing it to MP4 it says VFR. So is really only a metadata.

Look at that video link. That is a constant frame rate video, but

It is not and you showed that.
You showed not the framerate of your file but your projectsettings and your preview sees it mostly as a mediate, this time 59,999.
Your mediainfo file shows that the framerate is not constant.
To get that these are, I think, my most important settings to achieve that.

Choose above the best and fastest option you have (if)

I think this one is the most important.

In the video is a different video added with a CFR. And look at OBS forum. Every video recorded by OBS with simple settings is CFR. Just the metadata are different because of remuxing.

That is not necessary true with all media. Mediainfo reports specs that are hard boiled by definition of the specific codec. Another example is CFR renders from Handbrake 59.940 h264/x264 which reads VFR in Mediainfo, but the reported min/max fame rate reports the same indicating it is CFR. So, I'm not so sure that Mediainfo is reporting your source incorrectly as it show a difference between the min/max frame rate.

However, regardless of that, if you convert your source using Avidemux with a constant frame rate template, then trying that in Movie Studio, you will for sure determine if it has any bearing on you preview speed, thereby improving your preview or eliminating that from your issue.

BTW.: In the OBS forum i found this : For some reason the media info for OBS MP4s (that I've seen anyways) is always variable, even if you recorded at constant.

That is not true, you can beter look at this forum and here, but in this MediaInfo you can see my OBS made video is constant framerate.

Because you dont remuxing it from FLV. In the video link i added this :

General
Complete name                            : D:\Videa\Moje\Hotové\Apex Legends - Epizoda 3.mp4
Format                                   : MPEG-4
Format profile                           : Base Media / Version 2
Codec ID                                 : mp42 (isom/mp42)
File size                                : 1.82 GiB
Duration                                 : 10 min 38 s
Overall bit rate mode                    : Variable
Overall bit rate                         : 24.4 Mb/s
Encoded date                             : UTC 2019-03-11 20:51:53
Tagged date                              : UTC 2019-03-11 20:51:53

Video
ID                                       : 1
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.2
Format settings                          : CABAC / 1 Ref Frames
Format settings, CABAC                   : Yes
Format settings, RefFrames               : 1 frame
Format settings, GOP                     : M=1, N=30
Codec ID                                 : avc1
Codec ID/Info                            : Advanced Video Coding
Duration                                 : 10 min 38 s
Bit rate                                 : 24.2 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 59.940 (60000/1001) FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.194
Stream size                              : 1.80 GiB (99%)
Language                                 : English
Encoded date                             : UTC 2019-03-11 20:51:59
Tagged date                              : UTC 2019-03-11 20:51:59
Codec configuration box                  : avcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 10 min 38 s
Bit rate mode                            : Variable
Bit rate                                 : 253 kb/s
Maximum bit rate                         : 449 kb/s
Channel(s)                               : 2 channels
Channel layout                           : L R
Sampling rate                            : 48.0 kHz
Frame rate                               : 46.875 FPS (1024 SPF)
Compression mode                         : Lossy
Stream size                              : 19.3 MiB (1%)
Language                                 : English
Encoded date                             : UTC 2019-03-11 20:51:59
Tagged date                              : UTC 2019-03-11 20:51:59

But still lagging when text showed up.

I tried recorded a new video directly in MP4 so it is constant 60 FPS and problem is still here.

@Bazim I cannot determine which text you are using from the screenshot you provided, so I will ask, what text/title are you using when it slows down and how much does it slow down. Also, when you converted to the constant frame rate you posted, it is 59.940 fps, did you also match that in your project properties?

When Movie Studio is installed, the settings are pretty much optimized for most systems. I would perform full reset of Movie Studio. Hold down the 'Ctrl+Shift' keys and double left click on the Movie Studio desktop shortcut. That should pop-up a small window. Check the box 'Delete all cached application data', then left click on 'Yes'. This will reset Movie Studio to the installation default settings and delete the pesky cached application data that can sometimes cause Movie Studio to do strange things. If in the course of you attempting to get better playback settings were changed that may actually hurt that effort, this will set everything back to normal.

It may be that your system just isn't robust enough to run preview full speed when the play head goes through various FX's, transition, titles, etc. when processing 60p media. If that is the case you may want to consider converting your source material to Cineform intermediates, which usually will play easier than the typical h264 encoded material found in mp4 wrappers. Intermediate files are much larger than the OBS generated files, but sometimes that is the cost of get better playback speeds.

I don't think my PC is not good enought for not lagging a preview window. i7-5820k @ 4,5GHz with GTX1080 is pretty powerfull in my opinion.