I think the slowdown is caused by the blocked emitters UI, which needs to look at all the objects in the scene to determine which ones have emitter materials applied, so it can populate the lists. However, I've made a test with 8000 objects and several materials and I didn't get a noticeable delay when opening the properties window, so maybe there's another factor involved besides object and material counts. It would be helpful if you could send me a scene where the slowdown is obvious, so I can confirm that the blocked emitters section is the culprit.
Anyway, we will move that part of the UI into a separate dialog which opens when you click a button, so you only have to wait for it to load when you actually need to manipulate the emitter lists, not every time you need to access object properties.