ahp.. It could be not enough memory and its offloading some of the game into your page file and since its an HDD it needs more time.  when your running the game look at your available physical memory.. if its extremely low then thats probably your issue.

Otherwise, with your set-up I would not think you would be having a problem with BF3

If your computer is running low on memory is starts "page file" on your hard drive... essentially it is using your hard drive as memory.  Windows alocates so much HD space depending on its settings. 

Open your windows task manager when you have the game running and look at the performance tab. 

The physical Memory is your actual RAM.  How much is available compared to your total??

The PF usage is how much space on your HD the computer is using as "memory".  Many programs have things that default into PF instead of memory so that may be high. 

Just because you alt-tab out of the game it does not remove itself out of the physical memory so alt-tab when the game is open and look at the performance of your comp.  Alt-tabbing out of the game only causes your computer to stop processing the game temporarily in most cases. 

I wont be on mumble until at earliest sunday.. building my comp tonight and going to the maryland football game tomorrow early and gonna be out drinking all day sooo.. I can try to help on here when I can but I bet squishy would be able to help you on mumble.