Global fog decrease the fps ??

Hai to everyone...i am using global fog image effect from Image Effects asset in unity pro version....after adding global fog my fps is decreased and came to 20 to 25 before adding fog i am getting fps of above 55...am enabled HDR mode and using deferred rendering path....how can i improve the fps rate with global fog effect??? now i wondered how subway surfers and some other games using fog effect and eager to know about that??? Thanks in advance....