I downloaded pm 5 and I like the new look a lot. One small thing I discovered: When in 2-pane view (both in browser and recent mail window) and showing HTML, when clicking the sidebar on the drag-button the cursor sticks there. It needs another click in the window or the sidebar to release it. (The scrollwheel is still inoperative in HTML mode)
No performance problems on my G4 450 MHz cube nor on my 400 MHz G3 powerbook (both running 10.3.3) I get a fast response on all actions. Karel Gillissen