Here’s a quick tip to speed up Lightroom on an intel Mac. Select the program in the applications folder (single click), and then go to File>Get Info. In the General Panel, there is a checkbox to run the program in 32 bit mode. This is the default setting. Uncheck this box, and Lightroom will now run in 64 bit mode, speeding things up.
This is not available for G5 or earlier computers, nor for OS prior to 10.5.
On PC, you need to be running a 64 bit OS to install the 64 bit application.