I looked a while ago and it seems to be due to some of the webextensions I have installed, probably uBlock Origin but I can't be sure (28 extensions). Basically some things get cached that don't need to be, and then the cheap 5400rpm disk drive makes the browser stutter swapping between tab content processes.

I'm waiting for the hard drive to die so I can justify getting a new SSD or laptop, which I'm pretty sure will solve the problem, so no, I haven't filed a bug.