Well I cannot say for sure. But I believe rendering issues are to blame. I try it with IE9 and its slow to load.
But If I use Google Chrome which is uses the same rendering engine as Safari "web Kit" it loads much faster.
Same thing was happening with Mobileme too. Let's face it Apple basically ignores the other browsers compatibility with its apps. If it works fine if not that's fine too. I would try Google Chrome and although I am not a fan of Safari on a PC you could try it to see if its better.