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It's not that I don't consider it an issue, but I can't
do anything to improve the performance of the Tk text
widget.  IDLE is pure Python.

One thing that comes to mind is to set a maximum line 
length.  If the line exceeds that, print line(:(MAX -100)
+ '...' + line(:-100)  instead of printing the whole thing
which no one wants to look at anyway.

Another thing I've wanted to do is provide the ability
to clear the shell window when it gets too full, w/o
restarting IDLE.

Yes, you might ask the tkinter guys on their mail list,
I'd be interested in hearing their reply.