Since the original Experiencing performance related issues in Lightroom 4.x thread is now a whopping 43 pages long, and many of the original 4.0-4.2 performance issues have since been resolved, it's impossible to figure out who is still having problems, and what they can try. I'm therefore locking that thread to new posts, and putting a link to this one.
If you're having problems in 4.0 - 4.2, please update to 4.3. Here are the links: Windows - Mac You may also be interested in these Performance Hints, many of which were gathered from the long thread.
If you're still having problems with 4.3, please then go ahead and post below. Don't forget to include details such as your system specs (OS, processor, RAM, graphics card, etc), camera model and whether you shoot raw or JPEG, and what specifically you're finding slow (and ideally some timings on how slow). It's also useful to include notes of things you've tried to fix it.,