Well, i found something interesting :
- I have put all forest objects on separates layers,
- I have turn off all of them and the problem disappeared,
- I have turned them on one by one to isolate the forest object wich make 3ds max bug, freeze the time line and increase the time to save the file,
- I have four différent Forest objects that make it slow and all of them are Lawns or Meadows presets (Cut grass field 02 large / Daisy01 large)
The variation of global density doesn't work for those object but affect all the others (global). It's ok with proxy and decrease Max items in viewport instead.
Perhaps it is normal but i suggest to make lighter previews of lawns and meadows for large scenes?
Time lapse freeze when moving, select caméra bug, extremly slow saving scene are fixed.
Hope all will be fine now.