What are the specifications of your Mac? (what model, how much memory, etc.) We would assume you are running OS X 10.7 (Lion) - but regardless, is the operating system updated to the latest (in the case of Lion - 10.7.3 would be the latest).
Not knowing what model you have, or how much memory is installed leaves us sort of hanging on that regard - Any computer with 4GB or less is asking for lags, hangs, and general frustration when doing anything beyond reading emails. That is the game regardless of operating system.
Also - be certain that you have the latest version of Lightroom.
I have no idea - if you were asking me about a PC it would be no problem but I feel like I'm in a foreign country with a new language and strange surroundings. Where is that information located? I went to the launchpad and found a disk utility function but I'm afraid to do anything with it - what if "disk utility" translates into the same PC phrase as "format hard drive" ???