And MediaInfo for your files-what exactly are you editing?

Here are the screenshots j-v asked for.

As for what I'm editing, I recorded gameplay before I knew what I was doing using OBS studio, and the files ended up being in flv and mkv format. I've had to convert these to mp4 using Shutter Encoder. (H.265)

- For normal previewing in the Preview screen your Dynamic RAM Preview max is rediculus high. For that task it can be set to the default 200 or 0
- I cannot say much about the I/O setting because I use there QSV
- The driverversion you are using is not the best for Vegas Pro, therefore you need the Studio driver 461.72 as adviced by the Vegas Development.

Is there a reason you have legacy AVC and HEVC decoding checked? If you use that you won't be able to use GPU decoding with your card. (If the hardware decoding doesn't work well you can turn it off. Enabling legacy decoding means you can't open certain formats.)

After changing the settings you all recommended, it somehow runs smoothly again. (I didn't even change the drivers lmao)

Although, in case there's something with my file I've unknowingly messed up, I'll post some MediaInfo anyway just to ask you if this is a codec that actually works well with Vegas? (After all, there could be more problems uncovered as I go...)
And well, since it is a gaming video, there's a lot of clips and different small bits of media involved many sometimes played over eachother.
But, here's the MediaInfo for one file. Because I don't know how else to do it. lol
 

General
Complete name                            : E:\Workspace\Overwatch\Editing Batch 5\1 CH\2019-08-05 10-54-05_H.265.mp4
Format                                   : MPEG-4
Format profile                           : Base Media
Codec ID                                 : isom (isom/iso2/mp41)
File size                                : 34.2 MiB
Duration                                 : 53 s 22 ms
Overall bit rate                         : 5 417 kb/s
Writing application                      : Lavf58.51.101

Video
ID                                       : 1
Format                                   : HEVC
Format/Info                              : High Efficiency Video Coding
Format profile                           : Main@L4.1@Main
Codec ID                                 : hvc1
Codec ID/Info                            : High Efficiency Video Coding
Duration                                 : 53 s 0 ms
Bit rate                                 : 5 149 kb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 60.000 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.041
Stream size                              : 32.5 MiB (95%)
Writing library                          : x265 3.4+2-73ca1d7be377:[Windows][GCC 10.2.1][64 bit] 8bit+10bit
Encoding settings                        : cpuid=1111039 / frame-threads=4 / numa-pools=24,0 / wpp / no-pmode / no-pme / no-psnr / no-ssim / log-level=2 / input-csp=1 / input-res=1920x1080 / interlace=0 / total-frames=0 / level-idc=0 / high-tier=1 / uhd-bd=0 / ref=5 / no-allow-non-conformance / no-repeat-headers / annexb / no-aud / no-hrd / info / hash=0 / no-temporal-layers / open-gop / min-keyint=25 / keyint=250 / gop-lookahead=0 / bframes=8 / b-adapt=2 / b-pyramid / bframe-bias=0 / rc-lookahead=40 / lookahead-slices=0 / scenecut=40 / hist-scenecut=0 / radl=0 / no-splice / no-intra-refresh / ctu=64 / min-cu-size=8 / rect / amp / max-tu-size=32 / tu-inter-depth=3 / tu-intra-depth=3 / limit-tu=0 / rdoq-level=2 / dynamic-rd=0.00 / no-ssim-rd / signhide / no-tskip / nr-intra=0 / nr-inter=0 / no-constrained-intra / strong-intra-smoothing / max-merge=5 / limit-refs=0 / no-limit-modes / me=3 / subme=4 / merange=57 / temporal-mvp / no-frame-dup / no-hme / weightp / weightb / no-analyze-src-pics / deblock=0:0 / sao / no-sao-non-deblock / rd=6 / selective-sao=4 / no-early-skip / rskip / no-fast-intra / no-tskip-fast / no-cu-lossless / b-intra / no-splitrd-skip / rdpenalty=0 / psy-rd=2.00 / psy-rdoq=1.00 / no-rd-refine / no-lossless / cbqpoffs=0 / crqpoffs=0 / rc=abr / bitrate=5000 / qcomp=0.60 / qpstep=4 / stats-write=0 / stats-read=0 / ipratio=1.40 / pbratio=1.30 / aq-mode=2 / aq-strength=1.00 / cutree / zone-count=0 / no-strict-cbr / qg-size=32 / no-rc-grain / qpmax=69 / qpmin=0 / no-const-vbv / sar=1 / overscan=0 / videoformat=5 / range=0 / colorprim=2 / transfer=2 / colormatrix=2 / chromaloc=0 / display-window=0 / cll=0,0 / min-luma=0 / max-luma=255 / log2-max-poc-lsb=8 / vui-timing-info / vui-hrd-info / slices=1 / no-opt-qp-pps / no-opt-ref-list-length-pps / no-multi-pass-opt-rps / scenecut-bias=0.05 / hist-threshold=0.01 / no-opt-cu-delta-qp / no-aq-motion / no-hdr10 / no-hdr10-opt / no-dhdr10-opt / no-idr-recovery-sei / analysis-reuse-level=0 / analysis-save-reuse-level=0 / analysis-load-reuse-level=0 / scale-factor=0 / refine-intra=0 / refine-inter=0 / refine-mv=1 / refine-ctu-distortion=0 / no-limit-sao / ctu-info=0 / no-lowpass-dct / refine-analysis-type=0 / copy-pic=1 / max-ausize-factor=1.0 / no-dynamic-refine / no-single-sei / no-hevc-aq / no-svt / no-field / qp-adaptation-range=1.00 / no-scenecut-aware-qpconformance-window-offsets / right=0 / bottom=0 / decoder-max-rate=0
Color range                              : Limited
Codec configuration box                  : hvcC

Audio
ID                                       : 2
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : mp4a-40-2
Duration                                 : 53 s 22 ms
Duration_LastFrame                       : -13 ms
Bit rate mode                            : Constant
Bit rate                                 : 257 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                              : 1.63 MiB (5%)
Default                                  : Yes
Alternate group                          : 1

It is very likely that I've converted the files into some kind of unusable codec... I have no idea.

Nothing wrong with such a file in VPro 18, although if AVC it should be easier to decode , play and preview than the heavier HEVC that maybe are a little smaller.

Would it be a concern if Shutter Encoder took 100% CPU on my kind of machine trying to convert a file into a different format?
Because the only thing I found not doing that was the type of files I made when I cut them really short.

Your MediaInfo tells us little to nothing that can be assumed about the original .flv. or .mkv or .whatever video file you started with.

A file is always suspect if it is second- or third-generation because someone "may" have misencoded or misflagged it as something else along the way. Imagine that.

That's why we prefer you start with source source files, and ask questions to help us get you there.

Here's from the original .flv file:
 

General
Complete name                            : E:\Workspace\Overwatch\Replay Footage 2019-2020\2019-08-05 10-54-05.flv
Format                                   : Flash Video
File size                                : 1.28 GiB
Duration                                 : 6 min 5 s
Overall bit rate mode                    : Variable
Overall bit rate                         : 30.2 Mb/s
Writing application                      : Lavf57.84.100

Video
Format                                   : AVC
Format/Info                              : Advanced Video Codec
Format profile                           : High@L4.2
Format settings                          : CABAC / 2 Ref Frames
Format settings, CABAC                   : Yes
Format settings, Reference frames        : 2 frames
Codec ID                                 : 7
Duration                                 : 6 min 5 s
Bit rate mode                            : Variable
Bit rate                                 : 2 441 kb/s
Maximum bit rate                         : 40.0 Mb/s
Width                                    : 1 920 pixels
Height                                   : 1 080 pixels
Display aspect ratio                     : 16:9
Frame rate mode                          : Constant
Frame rate                               : 60.000 FPS
Color space                              : YUV
Chroma subsampling                       : 4:2:0
Bit depth                                : 8 bits
Scan type                                : Progressive
Bits/(Pixel*Frame)                       : 0.020
Stream size                              : 106 MiB (8%)
Color range                              : Limited
Color primaries                          : BT.709
Transfer characteristics                 : BT.709
Matrix coefficients                      : BT.470 System B/G

Audio
Format                                   : AAC LC
Format/Info                              : Advanced Audio Codec Low Complexity
Codec ID                                 : 10-2
Duration                                 : 6 min 5 s
Bit rate                                 : 125 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
Delay relative to video                  : 17 ms
Stream size                              : 5.44 MiB (0%)

Aside from not knowing what Shutter Encoder is, you've done a fairly good job.

Although, as @j-v suggests, keeping it AVC is what I would do.