Quoting Felix Blanke :

is nobody out there using xmonad with more then one screen and rxvt? If there are some ppl: Do you have that lag at the second/third screen?

I have no lag on my second screen on a variety of systems (all using proprietary nVidia drivers + TwinView). There's a lot of possible places to point fingers here: 1. bad display driver/xorg interaction 2. your shell / your local shell config / your system's global shell config 3. extensions to urxvt 4. extensions to xmonad that you're using 5. xmonad itself 6. probably others For debugging, you could consider trying the same rxvt test in other window managers, trying the same thing with the default xmonad config (e.g. main = xmonad defaultConfig), testing what happens when you disable one or both of the extra monitors, or checking what happens under the default shell configuration. Do let us know if any of these turn up anything interesting. It's a very strange problem. ~d