Recently, after an XFCE update on my Xubuntu I’ve noticed that switching windows via Alt-Tab takes too much time (up to 10 seconds). That is really annoying; especially because this time is wasted on fancy menu rendering. Switching with mouse click is as fast as before.
A quick glance at forums yielded no result except for old threads. I’ve found solution by educated guess (probably it is already posted somwhere).
So, you need to go to Settings — Window Manager Tweaks and at Cycling tab turn off “Draw frame around selected windows while cycling“. And that’s it!