Some LR 5 users, myself included, have reported a huge slowdown while just going through a preview of the photos in the library module; still present in version 5.4 (Mavericks 10.9.3)
It seems it's a bug related with big amounts of develop presets. So, till almighty sloppy dog adobeeee decides to fix this bug, a workaround is to (please archive first) delete as many as possible. I confirm this is the case. Don't expect miracles, but performance wise you'll notice an increase right away.

Some Windows users had same bug showing differently, when trying to undo, LR applied a preset instead.
SAME WORKAROUND in those cases too =)

Little rant, sorry if you're dating an adobe employee :P

I've been a LR user since version 1 and I don't really know what's going on with the preset indexing or the previewing of images, but since LR 4 it's an awful mess making zero difference being the catalog big or small. That said I’m tired to be continously forced to do things out of my pace and to upgrade / update and find fixings for what seems the pandora box of bugs. For me Lightroom has become very very inefficient, as much as I like grampas, you'll raise hell on earth if you put a 95 year old pot lover directing air traffic at the control tower of a major airport. This is how I feel these days working with LR.

Mavericks doesn't help and neither disabling compressed memory (´sudo nvram boot-args=”vm_compressor=1″´) – actually worsen results – neither purging the memory (´sudo purge´) have had any palpable improvements. So as much I dislike the idea of having yet another step in the process, I started to seriously test Photo Mechanic (which is like the Millenium Falcon to public transportation) and Darktable 1.4.2 (free) as folder based raw image viewers. It has come to a point where I admit that if I hadn’t +35K photos catalogued with LR I’ll switch to other platform… there are certainly better colour engines out there, specially for µFT raws ,-)

That's all folks, viva la amistad!!