"VP15" will not allow You to see the difference, although it is present. Maybe later I'll write more detailed , but for now here is an example: my 1-minute video "4K 60p" format w/o using effects, transitions and stabilization encoded in VP15, using XAVCS coder and my 8-core processor (see my profile) 5 min. 54 sec., loading all 8 cores at 100%. When I use the encoder "NVENC" and graphics card "GTX 1080", the encoding time is the same - 5 min 54 sec but the workload of all cores of the CPU drops to 20 %. 

Again, for comparison: 100$ "TMPGEnc Video Mastering Works 6" (for example old ver. 6.1.5.26 but not latest!) does the same job faster, and with better quality, using ALL AVAILABLE types (GPU, CPU, AVX1, AVX2,QSV but i can't use Intel QSV, cos "Broadwell-E" isn't contain GPU core) of hardware acceleration, in just 40 seconds!!! With a maximum load of CPU and GPU 

But it have poor editing functions 😨
Here's a marketing.😏

But everything changed dramatically as soon as you start using effects, transitions, stabilization. They actively use the hardware acceleration of the graphics card.

but on the other hand, all these transitions, effects and faders is a few percent in comparison with the duration of the main video and it's rendering

P.S.

In short  -  marketers won again technologists 😠

...the new Magix AVC codec in VPro 15. More fair you tested that one! Did you? ...

I guess that's what he means by "the encoder "NVENC"". What else could it be?

...The Magix AVC NVENC codec for that format gives a 600% faster render at my VPro 15...

600% faster than XAVC-S?

What a nice garden! Enjoy your mackerel.

Pls share Nvidia GPU models and drivers for your tests.

"VP15" will not allow You to see the difference, although it is present. Maybe later I'll write more detailed , but for now here is an example: my 1-minute video "4K 60p" format w/o using effects, transitions and stabilization encoded in VP15, using XAVCS coder and my 8-core processor (see my profile) 5 min. 54 sec., loading all 8 cores at 100%. When I use the encoder "NVENC" and graphics card "GTX 1080", the encoding time is the same - 5 min 54 sec but the workload of all cores of the CPU drops to 20 %. 

Again, for comparison: 100$ "TMPGEnc Video Mastering Works 6" (for example old ver. 6.1.5.26 but not latest!) does the same job faster, and with better quality, using ALL AVAILABLE types (GPU, CPU, AVX1, AVX2,QSV but i can't use Intel QSV, cos "Broadwell-E" isn't contain GPU core) of hardware acceleration, in just 40 seconds!!! With a maximum load of CPU and GPU 

But it have poor editing functions 😨
Here's a marketing.😏

But everything changed dramatically as soon as you start using effects, transitions, stabilization. They actively use the hardware acceleration of the graphics card.

but on the other hand, all these transitions, effects and faders is a few percent in comparison with the duration of the main video and it's rendering

P.S.

In short  -  marketers won again technologists 😠

1. Did you use XAVC to encode in both programs? Not all formats can take advantage of GPU rendering, though I'm not sure if TMPEG supports GPU accelerated XAVC rendering or not. I do know Vegas does not, CPU only.

2. For a better comparison, as mentioned above, encode to AVC on both. Use the 4K60 NVENC preset using Magix AVC encoder in Vegas 15, then do a comparable AVC render in TMPEG.

If it is at all possible to GPU accelerate XAVC, that would be nice, though it wouldn't matter to me as I never use that format.

About XAVC-S encoder, it works exactly the same, given the different duration of our video clips.
About NVENC
1. What a strange idea to use a template 25 fps. for video 60 fps. if you already have a preset for 59.94 fps.?
 As a result, you will get exactly 25 fps. video. Of course, the size of this video is almost three times less than the size of my video, for the same duration. Additionally, your video is initially stabilized by gyroscopes of the Quad and is easily compressed by the encoder.
 

Can you show us the media info of this created video?
2. Bitrate. How will looks video 4K 60p, with a bitrate of 25 megabits, which you choose? Where to watch it? On the smartphone? However if the source is a quadcopter's camera a smartphone is optimal for watching this video.
3. We have different video sources.
My camcorder shoots video in 3840x2160, with a fixed frame rate at 59.94 fps., with the bitrate of 135 megabits in "Canopus HQX" 😒 format and in AVI container!(😠)

Therefore, our results cannot be compared.
About the video card driver from nVidia then you are right. Not all drivers work properly in "VP15". 
Now I trying to find the optimal driver for Windows 7.

Thank you all for comments and forgive me for my bad english.

About XAVC-S encoder, it works exactly the same, given the different duration of our video clips.
About NVENC
1. What a strange idea to use a template 25 fps. for video 60 fps. if you already have a preset for 59.94 fps.?
 As a result, you will get exactly 25 fps. video.

He changed the fps from 25 to 60 & encoded at 60fps