I noticed that Safari was getting a bit slow to load the Flash plug-in the first time in every browsing session. After a quick look in my Internet Plug-ins (/Library/Internet Plug-Ins) folder, I found two files related to the Flash Player. A recent one (Flash Player.plugin) and an old one (flashplayer.xpt). After removing the .xpt file, Safari become far snappier to load flash the first time.
I guess the .xpt file is an old version of the plug-in that I dragged with me during successive installs and migrations; I expect a lot of users may have the same issue. (I had the same problem on my home Mac and my work Mac).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090202105730889