The other known cause that comes to mind is having a large number of controls on the Window, but that tends to come with other telltale signs (you end up seeing each control pop into view one at a time).
I wouldn’t expect that to be too many, perhaps not the issue.
Try turning ImplicitInstance=false and Visible=false. You may be able to get an overloaded Opening event to fire before the Window is visible. This is an untested suggestion (I dont actually keep everything in the noggin, I usually test before posting if I can).
I would suggest turning on Code Profiling to see if the debugging tools we have can take the guesswork out of finding what’s being slow.