Just for fun I created a project with 6k objects, 22k events and 68k actions, all in a single eventsheet. I can tell that performance is tanking at this point, extremely so on the eventsheet. It's pretty much not useable at this point, as every action taken there takes a good 3 seconds. Just switching to it freezes construct for maybe 20 seconds. I'd argue tho that nobody is insane enough to have 22k events all on a single eventsheet.
On the layout view however I notice little impact, though it gets slightly laggy once I open up the folder in the project view that contains all 6k objects. About 0.2 seconds delay that goes away if I close the folder again. Very much useable. I've increased the objects to 12k even and now it certainly starts lagging more, but still not unuseable, and closing the folder also still fixes it.
I've been mucking around in the project a while, but apart from switching to the eventsheet I don't have any freezes longer than a few seconds, and even those are only on the eventsheet. So... I dunno.