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This is a known problem with Tk/Tcl/tkinter - large output 
scrolls slowly. It's not something that can be fixed in IDLE.  I 
tried it on Arch Linux - IDLE 2.5a0 - Tk 8.4.  250,000 character 
output not too bad , 25 sec, but 10,000 lines of 25 char takes 
over twice that long, so breaking the lines doesn't help.

I don't see any response problem once the output completes.  The 
situation is exponentially worse at 500,000 char.

What is your use case?  IDLE is designed to be an IDE.  Why 
output such massive data?

You may be interested in Squeezer, a Noam Raphael extension to 
IDLEfork.

http://sourceforge.net/tracker/index.php?
func=detail&aid=704316&group_id=9579&atid=309579

I haven't tried it myself, but it might be what you're looking 
for.