It depends on what widget set you are using, and how efficient its' I/O
is. For example, some toolkits appear to draw all of their menus into
off-screen pixmaps, and then do a simple copy to make the menu appear. This
will give "instant" response, as opposed to a toolkit which needs to output
the text each time the menu bar item is selected, which will appear much
slower.
I've noticed that the DECwindows and Motif toolkits are really good about their I/O. The Athena (Xaw) widgets are about the worst. I haven't used OpenLook over XRemote so I can't comment on how well that works.