I'm having the same issue, however it has been happening for the last several versions. I first noticed this issue after upgrading to 10.5.6 (Snow Leopard) shortly after it was released. No speed effect after uninstalling, wiping preferences, and reinstalling. I am running on a 2.4Ghz Core 2 Duo MacBook (Aluminum) with 2GB of RAM.
With each update I find that while the application is more stable, there has been no appreciable or visible speed increase.