that's more than odd. there is nothing that should be able to push your CPU to full load when hardware decoding is used.
can you run GPU-Z go to "graphics Card" and press on the question mark to start a test. after this the PCIe information can change and that number is interesting.
the taskmanager GPU load is unreliable just an extreme example for you:
https://abload.de/img/taskmanageriei29.png