Is High Sierra Activity Monitor accurate on a a MacBook Pro from 2013?
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.