Processor usage near a hundred percent is not inherently bad. Exporting video or compiling code should use everything available; that is the hardware doing its job.
The problem is unexplained usage. When a process you did not start consumes cycles for hours, everything else queues behind it and the machine feels broken.
Common culprits are legitimate: search indexing after a large file move, antivirus scanning, update services, and cloud sync clients catching up. These finish if left alone.
Sustained load also creates heat, and heat causes the processor to slow itself deliberately. That is why a long-running background task can make a machine feel slow even after it ends.