Poor performance when handling instances in editor

[RexTheWho]

If you make a map that uses lots of instances (30+) that are activated on startup or at the same time the performance dip between the editor play test and normal play test is very noticeable, sometimes even leaving levels as a slideshow for around 10 seconds.

This seems to been a theme with instances where at some point menus and instances in general have been running poorly with instances but only in the editor.