It seems like it may be machine-dependent. Here, too, LR 5.6 is noticeably faster than 5.5 (Win 7/64, 16G Ram). All versions up to 5.4 had been fine on my machine, and 5.5 was just unbearably slow (made me feel as if I had 1 gig or Ram instead of 16). With ver.5.6, it's back to normal (and it seems like it's actually more responsive than any previous ver.5.*, although I haven't had time to check every single process, so don't know) -- no complaints so far.