This was driving me crazy. A week or so after updating my MacBook to Lion, Safari suddenly starting running like an absolute dog turd. So I've been using Chrome ever since, but I really prefer Safari, and the new Lion features are nice.
This evening I finally got round to doing some cunning Googling on the matter ("mac lion safari slow") and lo and behold, the
very first result provides what so far appears to be a complete fix.
If you have the same problem, don't follow the instructions there, however, since they are actually slightly more complicated than necessary. Here is the better fix:
1. Quit Safari if it is running.
2. Activate Finder.
3. Go to the "Go" menu.
4. Hold option key. "Library" will appear. Select it.
5. Navigate to Caches/com.apple.Safari.
6. Delete the "Cache.db" file.
That should do it. Selecting "Empty Cache..." menu in Safari will not suffice, you have to follow the above instructions.