Thank you for the help everyone.
The problem though is that the problem isn't restricted to the browser. It's the same on MegaSync, and the same even on JDownloader. Always the same pattern: a couple seconds on decent speed, then drops to an agonizingly slow speed, then rises to something between 100 and 500kbps for a couple of seconds and then drops again.
In the weeks before this, Kaspersky Internet Security was throwing some warnings about Mega.nz browser connections (not necessarily bad, but needed my permission). I recently disabled Kaspersky protection for testing download speeds but got no difference.
Perhaps I will route my connection through my cellphone (4G) to see if it's an ISP or IP thing. If someone got any other ideas please let me know 🙂