Sorry to only come back to this now.
I went through the browser, flushed my cookies and my cache, and after that the PMs open at normal speed again.
So, I guess it may actually have something to do with the browser (Firefox) and less with E.
At least it seems to hint towards that?
I don't really know much about browser architecture and how it interacts with websites, unfortunately, but I think it means it's on my end.
Thank you though for helping, Vek, I appreciate that :)