CPU AND GPU Usage lowering to 10-20 after playing 30-60min

[Mark3003]

For example on Overwatch in the middle on the most intense fight you can get with all the ults my GPU usage is around 50-60 and CPU around 40-50 and my fps doesn't drop below 150 (usually in the high 200+). but after playing few games I realize that my CPU GPU usage is both around 10-20 and in big fights they usage doesn't go up, therefore my fps drops down to the 60's...

What might be the problem ?

Edit : CPU and GPU temps are around the 50's when having 50-60 GPU and CPU usage. same when 10-20GPU CPU usage...