Thanks for the information, the only bit that could affect the speed from what I can see is the upgrade, which is dependent on what modifications were made during the upgrade. However, in order to get to the bottom of what is going we are going to need more information.
If you have firebug installed, can you go to the administration area, then open Firebug and enable the net panel. Once you have done so can you click on the home link and share the results here. This will give us a fair bit of information on what happens when you click on the link, as depending on the modules you have installed one of many things can be happening and we just need to narrow down what it is.