not sure if this is your problem ,

but do note that diablo 3 doesn't completely load all the data needed in the loading screen ,

it loads it while playing , therefore during the first 5 minutes of playing the game , u'd experience major drop in FPS due to the HDD limitations ,

a user has posted a guide on how to move the data to an external 16gb thumbdrive to eliminate the lag in the start .

or an alternative is to install diablo 3 on an SSD