NVIDIA 750 card is below recommended minimums for VP18+.

Is Vegas even using it or just your iGPU? How are you rendering? Your post is very vague.

@Lewis-Evans

"It runs at the edge of overheating at 70 deg C."

That has nothing to do with Vegas Pro. Run a synthetic tool (PRIME 95) that will maintain the CPU cores at 100% and fix your cooling before you get to the software.

70° C is at the edge of overheating?

Thanks for the tip; I'll never own one of those.

70c is over heating. DONOT do any CPU stress test because its will absolutely positively will blow up your PC.

An i9-12900K has a Maximum Junction Temperature of 100 degrees Celsius. That being said, if you do not tamper with configurable safety margins in BIOS, by default the offending core (or cores) will individually start throttling down well before reaching this temperature. Very short duration spikes up into the 90s are ok, sustained however should be kept below 80°C, not for the processor, nor for the chipset's sake, but motherboard components around the CPU do not like radiated heat above 80 degrees.

I did some further research and testing. It all works around the GPU. The Nvidia option on rendering takes the CPU down to about 12 percent. Thus the temperature drops dramatically. Render times are lower. Interestingly any changes were reflected into version 16. Which will cut the render times in half. Not a big deal right now because just doing four minute clips.. These later versions seem to be less efficient and more bloated.

I also disabled the e cores did not help on high load temperatures. Ran slightly faster due to 200 Mhz speed boost. But not worth the trouble go and do all that.

Actually it's 80 degrees C for the point where things start crashing. This one was running at 77C. Nividia option takes it down to 35 or 40 C. I have things stabilized to where they don't crash now.

I have my eyes on a rtx 3050 video card I'm waiting until the prices come down. Same thing on getting more memory.

Some time ago about version 13 when i got my 6700. That was when I put aside the video card and used the 630 graphics. Didn't know I was doing myself a disservice. Just thinking of the power usage. I was getting temperature spikes. They didn't show up on the temp display. I was getting a lot of crashed renders. Especially converting to different resolutions. Don't think 4k was in yet. I wound up with this liquid cooling system. That solved it.

I left something out: When I was trying to use Nvidia option I kept getting an error message mentioned something about driver. I checked the driver updaters and Windows - said they were up to date. So I went Nvidia website found a gamer driver a year newer than the one I had installed. The Installed one dated Oct 2020 Wasn't even a Windows 11 driver. Windows fought me installing the driver but I got it. Low and behold it worked with the option. That's where the revelation happened. I had this problem with windows 10 also with this video card. Why I went to 630 Graphics' on processor chip. I think that was a break even doing it either with GPU or Processor. My 4790 Processor and Z97 the processor is faster than the 530 GPU by about 30%

It would be to have something that just worked. But we wouldn't lean anything. All my learning has been from broken things.

@Lewis-Evans Hi, as you say you've got your eyes on a 3050 but why if you've got a 'newly constructed windows 11 12th gen intel machine 12 cores' did you put an old underpowered card like the Nvidia 750 in, ?

Use Help in Vegas - Check for driver updates or try Geforce Experience, that'll let you choose between the Studio or Game driver, it's generally recommended on here to use the Studio driver, I don't know if this will work with a card from 2014 tho, https://www.nvidia.com/en-gb/geforce/geforce-experience/

I'm sure someone will correct me but a lot of software has moved from using the CPU so much, to using the GPU or RAM more, because those two are easier to upgrade than your CPU... , Poss why 16 works better on your system?

Studio driver doesn't exist for pre-1XXX cards as far as I'm aware.
If the 750 doesn't work in 20 does the new CPU have an iGPU? QSV can also render quite quickly.

I'm not sure the OP has a question at this point, but my advice would be to get a modern GPU (might be better with a used 2070ti or 2080 than a new 3050?) and ensure there are no thermal issues no matter what the CPU is doing. You should be able to slam it and not have it throttle as this is a desktop.