[xmonad] Firefox redraw

The recent urxvt slow rendering thread reminded me that xmonad sometimes suffers from an interesting redraw slowdown with Firefox that doesn't happen in dwm or spectrwm. Sometimes Firefox as the sole window in a tag/desk takes longer to redraw itself upon entering the tag/desk when the previous tag/desk was an empty one. If you switch from a tag/desk with one or more windows, then it's quicker to redraw. Put differently, you either notice the redraw or you don't. I say sometimes because I haven't been able to reproduce it today, but I wonder if anyone else has seen this. The config is xmonad-testing/example-config.hs or my extended version with personalized settings, and I use compton to enable vsync, if that matters.