Why does my framerate drop when looking in certain directions?

[schecterplayer]

i was playing borderlands and noticed that when i looked a certain direction, the framerate dropped dramatically. like 60 to 30 fps. then i looked te opposite direction and back up to 60. why does this happen? it happens in a couple other games as well.

 

[Barritio]

Likely because the low fps direction has more detail / more to render / further draw distance, so the more it has to do, the more resources are required, the longer it takes, and the slower the frame rate.