It seems like every day there's a post about this exact thing, and I think Rooks01 and Latcady pretty much covered the reason.

In the end it really is all about the fact that it's an EA game and unoptimized - or more specifically not optimally optimized. Some computers seem to run it just fine, mine does and I've got an older setup, but some people have state of the art machines and still struggle to hit 40-50 fps.

Just tuning the graphics settings a bit helped me (biggest help was to turn shadows down), plus the double alt-enter trick which apparently forces the game into true fullscreen mode. Yes, it's as simple as it sounds. Hit alt-enter, you'll go to windowed mode. Hit it again, you'll go to fullscreen mode.