MacBook Pro Activity Monitor accurate?

I have an older MacBook Pro that generally runs just fine. But sometimes, it slows so much as to be unusable – to the point that I can type a couple dozen characters and sit back to watch as they take a full 10 to 15 seconds to appear on the screen.

Checking Activity Monitor, I can not find any problems, and in fact, see that the system and user are both only using 4% to 5% of CPU, leaving the CPU 90% to 92% idle – I have never sen it go below 86%.

Only 10 to 12 GB of the 16GB memory is being used. I am displaying All Processes and set the Update Frequency to 1 second. I am maxed out on the OS with High Sierra 10.13.6.

SimStreamProcessorService often shows up at the top of the list when the computer slows to the point of being unusable and after a Force Quit the computer will operate normally. For awhile.

Is there a reason Activity Monitor is not showing me the culprit? Is there something I can do to find it? How do I use the other 90% of the CPU???