In case this helps anyone, I spent this morning debugging my issue further and I am pretty certain it's a hardware fault either with, or affecting, the dGPU in my laptop.

If I force internal GPU with `sudo pmset -b gpuswitch 0`, animations and graphics benchmarks are much faster than with dGPU enabled! For example, I tried the Unigine Heaven benchmark and I get 14fps with internal GPU, 4fps with discrete (5300M) GPU, which I am pretty sure is not right lol. (If you run this command, `sudo pmset -b gpuswitch 2` set it back to auto).

Did a clean install of Catalina to rule out OS issues, and got friends with the same laptop to test out the same website I was seeing extreme slowness on (it was fine for them).

The laptop recently had a repair and pretty much every part was replaced as a result of liquid damage, so I suspect either a faulty part or incorrect installation. Off to the Apple Store I go!