I could not figure out why the hell Safari 5 was running so slow in my personal mac, when compared with the one i use for work. Until i read some nice tips on the subject, namely this one article.

To make the story short: I had installed (a long long time ago) a couple of plugins (not to be confused with the new extensions) and completely forgot about them. One of them was the cause for Safari to completely block for 5-10 seconds at a time and show me the spinning color onion. I don't care which one but, bottom line is, one probable cause for your problem is a misbehaved plugin.

The safest way to check is to go to Preferences -> Security and disable the plugins. Then restart the browser and check if the problem went away. If so, point Finder to ~/Library/Internet Plug-Ins and move the files to another folder. Enable the plugins again in the browser and restart. If it's still fast, then that was your problem. Now the only thing left to do is to re-add them to the folder one by one, until you figure out which one is making Safari suck ;)

Happy hunting :)