I've just gone to my config and set force-opengl to false, and this seems to have fixed the issue.
For anyone else having similar (unexplained) performance problems, that should probably be the first thing you check.

I'm getting significant performance issues / frame drops with my inventory open when it's even just around 50% full. The problems are even further exaggerated with a full inventory.
The issues are incredibly inconsistent - and even just standing still and opening / closing the inventory periodically ...