Delerium actually lasts a little longer than the tool tip suggest - its closer to 11 seconds than it is to 10.

Even with low skill speed (my GCD is 2.42) i can only really consistently get 4 GCD's in BW some of the time, not all of it. I dont know how anyone is expected to get 5 in it. The duration is either a miss-print, or there is some weird shenanigans happening with BW because it does NOT last anywhere near as long as Delerium.