Slow export. Low use of computing power.

The rendering uses other resources as well, most especially including memory bandwidth between the CPU and GPU.  That could be the limiting factor here.

You could also compare export performance with the option Use GPU For Export turned off.  When you do that, on most machines you'll see CPU usage between 80% and 95%.   But with newer GPUs with at least 8 GB of memory, it will be slower than using the GPU.