Because HEVC is a very compressed codec, NLEs such as vegas Pro usually have a much harder time processing it. There are several YouTube videos explaining HEVC - such as this one:
With 5.3K GoPro HEVC video, I experience the same sort of stuttering playback in Vegas Pro 21 as you are experiencing. The best way to get better HEVC playback in Vegas Pro is to create proxies and use the Draft or Preview playback settings so that the proxies are used for playback/editing rather than the original HEVC video. Or transcode the video to another .mp4 format that Vegas Pro and other NLEs are happier with - which I have just done.
It's not just Vegas Pro that has problems with HEVC, I've found that DaVinci Resolve Studio 18 also struggles with HEVC video.
Dynamic ram preview at 0MB hurts performance- put it back on the defaults.
What version of VEGAS is this? In VP 21 HEVC support is improving, especially as you are using NVDEC for decoding (I assume, check file io in preferences). In build 187 try with experimental HEVC decoding on and off and see if there's any improvement.
He needs to fix his computer, Like RogerS said, are you using VP21b187 and if so what decoder is in use, and post mediainfo for your file.
If it's using the new Vegas HEVC GPU decoder I would expect it to play fine on any half decent hardware. I did notice it won't play 6K smoothly with new decoder on Best/FULL, however it does play smoothly at Best/half which is a really good compromise
A couple of things...first, upgrade to build 187. The screen capture you provided appears to be 108. Second, please give a MediaInfo readout on the hevc media you are using to help us diagnose the problems you are seeing with preview performance.
@Bini Resolve is better optimized for Nvidia than Vegas which is more general purpose and better optimized for Amd and Intel graphics than free Resolve. Your setup with no Intel igpu and only an Nvidia Quadro may be great for Resolve but not so much with Vegas. If you dropped an Intel Arc in as a 2nd gpu, even a dirt-cheap a380, that would boost your Vegas hevc performance with vp20. Personally, I'd do that 1st. But if you did a vp21 upgrade too, performance would be even higher. And if you're running Resolve Studio, having an Intel for decoding would benefit that too.
So my experience with this is 1-3fps with the 1080Ti GPU enabled (basically no GPU decode). If I use the iGPU Intel UHD 770 (CPU graphics) I get 60fps at Best Full. This is with V20.411 My GPU is about 10% slower than the P6000. My GPU can decode H265 8bit, 10bit and 12 bit 4:2:0 but V20 is not using it to do so. When importing it into V20 I get a message that V20 is downloading the mainconcept HEVC decoder. Looking at the decoder being used it is so4compoundplug.dll for both UHD 770 and 1080ti so the V20 so4compoundplug only uses the 770 for hardware decoding this file.
@Bini does task manager show the P6000 as decoding the video?