Is High Sierra Activity Monitor accurate on a a MacBook Pro from 2013? 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 how I can use 90% of the CPU?
Unfortunately the answer is most likely &quot;yes.&quot;
There is a reason, or at least a possible to likely reason(s).
This could easily be bound to a failing component (logic board, drive, I/O etc.) As if there are hardware problems those won't show up in activity monitor. Ask someone who (very recently) knows.

Boot the Mac while holding the d to go into diagnostic mode. The Mac will do its thing and report any problems it can.
Boot the Mac into Recovery Mode by holding the ⌘r kees at boot.

Once in recovery mode go to Disk utility. Select the top drive (your boot drive) and click on First aid.
Both of those tests will find gross issues with your Mac. It is not a guarantee that it will find a problem with any hardware or soft errors/bad blacks on the SSD. But it is always with trying.
OK so we've tried diagnostic tools, how about a different user profile? Go into System Preferences, create a new admin user and log in as that user. Problem gone? The issue was something in your user profile. The fun part is finding what it was. And by fun, I mean no fun at all.
If nothing is found there. I would back up your whole drive (Time Machine or 3rd party utility) Create a flash drive macOS installer with your preferred OS version (there are tutorials all over the place on hot to do this). Boot from the installer, re-format your HD and install the OS fresh.
Once that is done boot your mac normally and before restoring any of your apps, docs, pics or settings see if the problem is still there.
If the problem still exists it is most likely hardware and time to either have it repaired or buy a new Mac.