Around here we generally recommend people use a dedicated QuickSync transcoder. It’s faster, better quality, more reliable, and usually cheaper. Even the most basic Celeron processor can achieve 20 1080p transcodes.
In my experience, Nvidia cards are still somewhat CPU dependent when transcoding, but it does look like you have an adequate CPU. How much RAM do you have?
As far as audio transcoding, that always takes place on the CPU, and currently is not hardware accelerated on any device.