Immediately after.
How long it takes to show the window.
Yes, that’s what it does 
Which, as I suspected, indicates that the problem is not in RT but somewhere else. Perhaps there’s something in your particular windows installation that doesn’t play nicely with GTK (the GUI toolkit that RT uses).
Bottom line: if the simple test exe becomes very slow when RT does, you should consider reporting this to the GTK devs and see whether they can tell you something…