Thanks for the replies :)
Just a few additional things:
- In OSX (and Safari), the URL itself actually shows in Activity Monitor. Most of the time (but not all the time) the process using the most CPU is browser-related. There are some other processes in the background from time to time though.
- RAM usage doesn't seem an issue, there's plenty of RAM free in OSX (and Linux, currently in Mint Cinnamon using 2.6GB).
- Plenty of space on the SSD, especially in the Linux install (separate SSDs).
I've tried uBlock in Firefox and it seems to help a bit, and I'll give Vivaldi, Opera and Brave a try as well. I tried Midori assuming it would be lighter, but it still uses a fair bit of CPU in some sites. The MMM forums are just fine, but other sites tax the machine a fair bit :)
It is probably almost time to replace the thing though. The bottom of the laptop gets very hot in normal usage, and while it'd be nice in winter, it's not great in summer. I had it on a desk at my parents' place when I was down there over Christmas and it felt like it was about to melt. :)