Explain what you're doing in the project in detail. I imagine it's waiting on GPU. I assume you're not using the MKV as your source

Rendering an edit made from multicam that uses 2 Vegas projects as clips:

MediaInfo report of the source footage which is used in the vegas projects.

I figured it has room to spare. Perhaps it is indeed waiting for the gpu? It's an RTX 3070

Maybe it's the variable frame rate. You could try testing that by re-encoding source files to CFR (software encode) and see if anything changed. Nvenc Hardware encoding has a degree of variability even when it says CFR. This is just to test

The point of GPU encoding is to offload computation to the GPU. It seems to be doing that. If you want to see the CPU work harder change the render from NVENC to Mainconcept encoder.

@Miles-Thatch Hi, Sorry i can't help but just thought i'd share, i was having a prob with rendering using (NVENC) so took these screen captures, the two i use are the Mainconcept Internet UHD 2160p 29.97 fps & the Internet UHD 2160p 29.97 fps (NVENC)

This is like RogerS says, the Internet UHD 2160p 29.97 fps (NVENC) is faster but calmer & looks like this when rendering

But the top one, Internet UHD 2160p 29.97 fps is slower but Max's out my CPU, the machine sounds like it's going to explode, every fan working at it's max 😮 it looks like this.

I haven't done anything with the settings on either GPU, CPU or Vegas,

This is it after rendering just for reference 😉

 

This is like RogerS says, the Internet UHD 2160p 29.97 fps (NVENC) is faster but calmer & looks like this when rendering

But the top one, Internet UHD 2160p 29.97 fps is slower but Max's out my CPU, the machine sounds like it's going to explode, every fan working at it's max 😮 it looks like this.

Your example is what you would expect to see, in the Nvenc example you're getting 65% use of a 32core CPU for processing without the encoding, but he's getting 11% utilization of a 8 core cpu. I could get similar low cpu utilization by bottlenecking the throughput with Vegas Denoise. It uses so much GPU it slows the CPU down due to delays in processing