1: enter the overview
2: click on the 'Applications' item at the top
  (there is a disconcerting pause)
3: click on the 'Windows' item again
  (no pause)
4: click on the 'Applications' item a second time
  (no pause)
5: exit overview and re-enter
6: click on 'Applications'
  (there is a pause again)

The shell appears to do some sort of caching of information when it shows the Applications list for the first time and hold on to this cache for the duration of the time spent in the overview, discarding it when leaving the overview.  Maybe that's desktop files being cached or just textures being uploaded to the graphics card.

Either way, the pause is a bit embarrassing, and clearly possible to avoid.  I wonder if there could be some precaching here?