Ok I made some more tests, sketchup doesn't bug completely, it just takes ages too delete everything, I've let my computer run for a couple of hours and when I looked back it was deleted so I don't know how long it takes exactly.
I'm working on huge projects modelled in archicad and saved in sketchup. Sometimes I get an update from the file from the client and want to replace the exisiting model with the new one, in this file just deleting the old model takes forever so importing the new one is also a problem.