Your situation is pretty complicated and vague to really troubleshoot unless you can identify the specific codecs on the timeline you’re trying to export.
Your CUDA card will only have a significant effect on a GPU-assisted effect, not on decode or encode.
I would say if your CPU cores look under utilized, that your system is waiting for data…is the drive all this is on full? Is it a small, external drive?
TimK,
Director, Consultant
Kolb Productions,
Adobe Certified Instructor