The impact of gpu preview acceleration on gpu renders is minimal because Vegas updates the preview at a lower priority than the render and will skip more preview frames during the render as it deems fit. The Vegas preview frame skipping also increases when doing background screen capture. I find that I get slightly faster renders, however, if I plug my monitor into my motherboard hdmi which is driven by an onboard igpu. I also noticed that when my monitor is plugged into my primary pcie video that I get slightly smoother plays and faster renders if the preview is set to one of the dimensions that turn its numerical indicator white. Such as 960x540 in HD or UHD projects. Generally an even fraction of the frame size with any of the preview-auto settings. Not a big difference. Maybe 2 to 5 seconds on each of the benchmarks.

GPU Acceleration of Video Processing affects only certain preview situations.

It has nothing to do with GPU rendering.

But why during rendering it so much slower with GPU acceleration off?

It doesn't affect preview only, e.g. it also accelerates processing of many of the FX. So if in your project you use many GPU supported FX, that could also accelerate part of the rendering (because the FX processing would not have been buffered before, except of some seconds from the RAM maybe).

GPU Acceleration of Video Processing affects only certain preview situations.

It has nothing to do with GPU rendering.

But why during rendering it so much slower with GPU acceleration off?

GPU acc. of video FXs is one thing, GPU encoding/decoding of video files is another.

Turn it off and CPU has to do it all. 😏

1, 
Your desktop is typically configed for gaming not NLE computing. 

In game all motionpics are ready with their assembling. You only need a powerful GPU to discharge/decode whereby the CPU helps. 
On timeline all motionpics are waiting for design that goes subsequent to your plan. The CPU works for that; the GPU helps by source decoding and/or some OFX-plugin assembling. 

2, 
The render process is not the discharge in gaming.

It's a process of the timeline assembling. Thus, the GPU still can only help in need. 
Because that 6-core Ryzen5-3600x struggles with its own task in limitation, it can give your GPU not much commanding. 

3, 
My CPU is powerful, but my GPU only so much about 50% as your RTX2060. Thus, it's very busy with all the heavy commanding from my CPU. 
I'm proud my concept works, is worth the invest. Sure, a smart Graka doesn't harm.  

4, 
Don't take much attension to those tachometers. Your can see the huge differences between the Windows' indicator and that one from AMD. (PS: and that one by VEGASpro.)
Diagram by Vegas Sampleproject rendering --

The dynamic RAMpreview is used for FX or transition, (say, 4-6sec. long) when the computing process can't supply real-time playback under certain preview resolution e.g. Half in 4K project (in this case Half=1920x1080).

Its also helpful if the hardware decoding lacks. In short, it's only needed when computing can't go real-time and you want it be.

I have 64GB installed but set my dynamicRAM "only" at 4GB. It can give me ca. 6sec real-time preview under its Quater-res. (=960x540. I edit in HDR10 environment and the button for HDR display window is activated.)

Thus, turn off the GPU-acceleration should not be considered in any situation, even if there's crash by rendering or other cases (in my opinion, they are all computer configs problems).

You are likely CPU bottlenecked with these effects and settings, and the GPU is waiting for frames to render.

@Yelandkeil I still don't get it,

if GPU Acceleration has nothing to do when exporting a clip, why when it was off, it significantly slow down the render speed like 3x slower? Plus from my taskbar the GPU do nothing.

When I turn on my GPU acceleration (That is used for decode clip in timeline, FX processing etc), then only it render much more faster. why?

GPU Acceleration of Video Processing ON

GPU Acceleration of Video Processing OFF

Both test I pick 1080p 25FPS NVENC template & customize to 24fps to fit the media.

Footage is Sony clip slog2 color correct with grading panel & got LAB adjust OFX

Project settings is normal 8 bit full range

You are likely CPU bottlenecked with these effects and settings, and the GPU is waiting for frames to render.

What do you suggest then? upgrade my CPU? Is Ryzen 7 with 8 core sufficient enuf?

@alifftudm95 Well, you are gaining a 1,500% increase as it is, it’s down to the projects media and fx to some extent, as well as your HW.

Try a test project and see how it goes, ideally where using CPU only already gives greater than fractional render numbers. Then add in Gpu.

@alifftudm95 just discussed this in another post.

your confusion rooted in the render template. if you use that one with NVENC, it's true that your GPU does the encoding job. Whether it's quicker or slower, that is another thing.

If you take the template without NVENC (hardware encode), the render process (all computing) will be taken by CPU alone, but the GPU helps by "hardware acceleration" when your FX-plugins on timeline support that (special computing).

2,

it is not the question which cpu with how many cores should be sufficient enough, it's how much will you invest: the ryzen5950x ca. 800euro, the Ryzen Threadripper 2990WX ca. 1400euro, haha never mind.😁

You are likely CPU bottlenecked with these effects and settings, and the GPU is waiting for frames to render.

What do you suggest then? upgrade my CPU? Is Ryzen 7 with 8 core sufficient enuf?

Just wait a few more minutes for the render to complete?
Wait for Vegas to be better optimized in the next version?

TechGage and the benchmarking continued threads has render benchmarks to see how different CPUs and GPUs compare.

@alifftudm95

From your own numbers, that represents 15-fold increase in performance. Not what you were expecting, but that seems a significant improvement.

So if in your project you use many GPU supported FX, that could also accelerate part of the rendering

Yes, "could" being the operative word here. In my limited experience, the real world advantage amounts to "not much."

@alifftudm95, would you consider doing me a favor?

There's a Stress Test offered by Musicvid here and I'd like to see hwo big is the difference between your desktop and mine.

Thanks in advance.