I've suffered from this for a long while myself. It's a window manager thing, but I haven't heard of anyone actually fixing it.

You can find more discussion (and window managers that don't have the problem) in these threads from comp.lang.perl.tk: DialogBox delay and Tk:Dialog slow.

In reply to Re: Why is the display of the contents of a TK Dialog delayed by TheoPetersen
in thread Why is the display of the contents of a TK Dialog delayed by busunsl

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