I have searched for this and couldn't find this. My friend in the beta said that when you start the game before tristram you are kinda laggy till you reach the gates... He has a top end computer and I was wondering if you think blizzard knows about this
I wondered this very thing for awhile now and I finally went back and double checked the known issues thread on Battle.net. It's there as a general issue, the game will try to reload assets it has already loaded, causing lag in game from caching data. So, in other words, the game is not fully optimized yet. They'll most likely have it sorted out by release.
This is actually a known issue that's supposedly fixed in the release build. I can't find the source atm, but I'm pretty sure Bashiok or Jay mentioned it somewhere.
Low fps on game startup?