I've isolated a problem with string objects with a lot of text causing massive slowdowns in MMF2 apps.
If a string object is in the frame and contains a large amount of text, it will slow down the application.
Even if the string object is invisible, or destroyed at runtime, or drawn as a background, it will still cause slowdown.
This example provides two frames, the first frame has no string object and should run at full speed, and the second frame is exactly the same but has a large invisible string object on the left side of the frame.
String Slowdown Bug.mfa
Edit: I've reposted this in the bugtracker: http://bugbox.clickteam.com/issues/636