I'm using Windows 7. The delay happens in all the pane modes and when you open a message in a single window. It also happens even if you change the default view to text only. It also prevents you from down-arrowing through messages until they load fully--either by delaying the move or going (usually temporarily) into "program not responding" mode with a grayed-out screen.

I tried safe mode and all the usual workarounds. A problem almost identical to mine was presented in the "answers" to my question above, but that person didn't get a real answer either. Reversion to 45.8.0 totally solved my problem, so something is obviously very wrong with 52.1.1. I've been a Thunderbird fan for years and have never seen this kind of slow performance, which renders the program well-night useless. Please fix it!