What Happens If My CPU is not good enough for a game?

I wanna play a nice FPS(First Person Shooter)game and i just went to a site so i can see which games will work.I played Battlefield 2 on my other computer and I like it so i decided to play it on this older and worse computer but it looks like i need 1.7 GHz and I only have 1.3 GHz (I don't have acces to my other PC so i must play games on this one)so I would like to know what happens.Same with Battlefield 2142 i need 1.7 GHz.Also 1.8GHz for COD4:MW.So i would like to know if these or any other games that need more GHz will work.