WebKit:
Top Processes Snapshot by Memory:
Process (count) RAM usage (Source - Location)
com.apple.WebKit.WebContent (13) 3.30 GB (Apple) ⬅️
is related to Safari and may be piggish due to one of these:
Safari extensions:
[Loaded] Day One - /Applications/Day One.app
[Loaded] safari-extension - /Applications/Capital One Shopping.app
[Loaded] Evernote Web Clipper - /Applications/Evernote Web Clipper.app
[Loaded] Capital One Shopping - /Applications/Capital One Shopping.app
Note that you have two Capital One shopping Helpers. Intended?
Test by turning off all extensions ( Safari settings > Extensions tab). If symptoms go away, one of them is suspect. Turn them on one at a time and test. If symptoms return, the last one reactivated is suspect.
One thing I like to do in diagnosing symptoms like yours is to test with any external drives disconnected, especially USB drives. Sometimes a drive or its cable can drag down performance. Easy enough to test. I seldom expect issue with an external SSD but would try disconnecting it anyway.
Your storage "read" speed is about 2/3 of what i expect. Even M1 Macs in 2020 could do 3200 MB/sec both ways. I've seen BrickMyMac slow drive performance, so the storage speeds may increase once you fully show the aftermarket dreck the door. Macs—catlike—need no outside help to clean themselves.