For a clearer picture, on the "Performance" tab of Task Manager, click on "Open Resource Monitor". You can monitor each thread created, memory and disk i/o alternately or open 3 instances and monitor all simultaneously.
I'm having the same issue on Camtasia 2020 for Mac – I'm on a 24-core Mac Pro and when encoding (exporting) a video, it's using 80% of one core, and running slow slow slow. It doesn't appear that there's a setting to adjust this. Any pointers?