Exporting Video Takes 95% of CPU Usade

While you're performing diagnosis on that PC, did you remember to completely uninstall the existing graphics drivers before you switched GPUs from one generation to the other? You see, it sounds like your system might still have traces of the Fermi-generation GPU settings in the Windows registry, which will not function properly on a newer-generation (such as Pascal) GPU. And don't forget to download the new drivers directly from nVidia and not through Microsoft Update. And be sure to use Custom installation and check the box marked "Clean installation" when you reinstall the drivers after swapping cards.

In addition to checking for that, it is very possible that your old GT 610 would not let you use anywhere near the full CPU utilization even if you wanted to. This is because the GT 610 is so horrible (it is actually a rebadged GT 520, with only 48 CUDA cores and only 64-bit DDR3 graphics RAM with a throughput of only 12.8 or 14.4 GB/s) that it actually degrades even the CPU-only performance, even in situations that make absolutely no use whatsoever of the GPU. As a result of that, your i5-6402P is so degraded at this artificially capped CPU utilization limit that it performs no better than a 10-year-old Core 2 Quad CPU or even a Core 2 Duo dual-core CPU.

I'm not sure I understand.  You're complaining that export is making use of almost all available resources?  You want it to use less, and go slower?  You want it to use 100%?  Help me out here.