Thank you for your replies guys. Good ideas.
I'm running a few scripts and extensions; the ones working on Gmail are Gmelius, Rapportive and μBlock. I tried disabling them (or running it in a private window - great suggestion!) and it seemed like things were a bit better, but barely.
Then I disabled the sidebar (which was only running a bookmarks and history extensions) and - wow, that made a difference! Strange... Disabling Rapportive also seemed to help. Still keeping Gmelius....