Sorry, i don't like it.

As fast as ram? In which format? I don't wan't to trash my free space.
It also means more clicks, because i have to press the render button every time i adjust anything on that small part of the timeline.

My real question is: why ram preview stutters, drop frames when the video has been already computed to the ram?

Before your question could be answered, 3 things should be clear:

1, source material

2, project settings and preview scale (resolution/quality)

3, dynamic RAM volume

All these indicate that your HW (especially 16GB) is tight for a modern video edit - 4k60p I mean.
Usually I never answer questions like why my car runs slow on Autobahn.
So please give enough info for a discussion.

I give you an example:

1, my project is always 4k60p with Good/Half preview; Resample mode: disabled
2, my dRAM is 30%=16GB
3, for cutting modus (8bitfull format) I get

a, ca. 27sec fullspeed playback RAMplayview with 5k60p 8bitHEVC source
b, ca. 1min40sec with 8k30p 8bitHEVC source (the RAMbuilding is slow)
c, ca. 32sec with 4k50p 10bit422HEVC source

Once RAMbuilding finished it runs smoothly; I also only need RAMpreview for such source materials.
Normally, I make swap files to avoid often RAMpreview.

Just forgot to mention, sometimes the dRAM cache could be "unclear", so select an empty place to build the dRAM preview for releasing the cache, then build the right dRAM for the wanted event.

Apropos HW, in ACES modus my machine can only give ca. 50-54fps speed with stills or plain text with or without dRAMpreview.
This could be the case for your machine, too.

@relaxvideo assuming you're talking about the PC in your signature and also not using the latest version of Vegaspro, I understand your concerns about SHIFT+M.

Very often when i make a few seconds ram preview with SHIFT+B playback stutters and only will be smooth after 5-6 looping

This happens to me sometimes if I have an Fx window open or other windows in Vegas. It also happens when the project is more complex with more tracks. The ideal is to turn on for a moment solo only the track where you want to use the dynamic ram. Sometimes restarting Vegas or the whole PC will help for a while.

It shows you how much is in memory at least in 19+. My guess is it is running out of room for the full 5 seconds.

@relaxvideo - Happy Otter Scripts KwikPreview is your friend. Use it all the time.

https://tools4vegas.com/kwikpreview/

@Grazie I don't recognize the added value of HOS kwikpreview against the renewed SHIFT+M pre-render option of VP20.

@3POINT - I recognise I can use alternative renderer MAGICYUV or Voukoder which are blisteringly fast 😀.

@3POINT - I also recognize, that through its Preview Software procedure I can get a split screen:

I can also "Save" the outcome as a Video file.

Why should I use Voukoder (or MagixYUV) to generate a preview? At my system a Voukoder render is always slower than a Magix AVC NVENC supported render (which is my default prerendertemplate).

@3POINT - You make excellent points!

Keep your general RAM usage as low as possible during editing, turn your internet connection off, and don't reserve more dynamic ram memory than you need for the preview. It may take a couple of playbacks to consolidate enough contiguous buffer space.

Thanks all of you for the tips, i will check them!

My project is complex, yes, a 2 hours wedding.
Source material is a 4k-25p h264 60mbit from ZV-E10 cam + an outlined text with default animation.

This was very stuttering so i like to see 5 sec from memory (ram preview).

Project settings is 4k-25p, preview scale is half good quality
Dynamic RAM volume: few gigabytes

Windows task manager showed around 10GB was used from 16. So i doubt its a memory problem.

The preview was smooth only after 5 or 6th looped playback.

It may be more of a memory allocation problem, which becomes trickier with the less RAM you have available. The buffers get fragmented and are constantly rearranging themselves to give you the most contiguous addresses for smooth playback. If 6GB is all you have left, it leaves little wiggle room for all other running processes to make adjustments.

Reducing system load and turning off the internet helps my system, which is wimpier than yours.

This is fundamental; a similar thing happens when you load a high-bandwidth video in VLC Player, for instance. You often see corruption and stalls the first couple of times through, and then it smooths out.

Your "complex" video project is already using a ton of Virtual Memory, which comingles hard drive space with available RAM. In lieu of my other suggestions, consider intermediate or conventional file prerenders.

Thanks for considering this scenario, and do let us know what you come up with ;?)

Your "complex" video project is already using a ton of Virtual Memory, which comingles hard drive space with available RAM. In lieu of my other suggestions, consider intermediate or conventional file prerenders.

Thanks for considering this scenario, and do let us know what you come up with ;?)

Just try in that case SHIFT+M (it's one click command in VP20, like SHIFT+B) uses just a fraction of drive space of what proxies or intermediates take, not talked about the time it needs to generate proxies or intermediates.

It shows you how much is in memory at least in 19+. My guess is it is running out of room for the full 5 seconds.

Where? I use V19 b643

@relaxvideo Take also in account that for 4k editing, at least 32 GB RAM is recommended.

But then why do i see 6GB free, I never understand that.
If i have few GB-s free, then total ram is enough, isn't?

Vegas just shows you the total available RAM of your system, doesn't mean you can reserve all RAM just for previewing.

i use windows task manager which show 6GB is still available. Why 64bit Vegas don't use?

I'm not an IT expert, but I can imagine that part of available RAM is needed for other processes on your PC and that you can not allocate all RAM just for Vegas. All this issues you don't have when you use SHIFT+M instead...