It's just a delay, but a pretty long one each time. Stoping the Web Shield doesn't help, but stopping the Standard Shield seems to fix the problem.
Extensions: Adblock, Roboform, CustomizeGoogle, BugMeNot, FirefoxShowcase, Flashblock, IEView, LinkVisitor, MenuEditor, ResizeableTextArea, Sage. Disabled MouseZoom and NoSquint.
Incidentally, another problem I'm having with Firefox lately is a delay in appearance of typed text onscreen or in response to things like right or left arrow movement.