Pretty strange, I usually use Firefox for my browsing needs but I'm now building a new website with a fresh WP install and I recently added the All-in-One Event Calendar plugin.
Haven't noticed anything wrong until I try to log in as admin using IE9. That was very slow! i tried Chrome, same thing but a bit faster. No problem with Firefox.
It not until I disabled All-in-One Event Calendar that speed was back to normal. Enabled it again and it was slow again.
Are you aware of this?