its gone very slow

You have not mentioned what type(s) of game(s) you are having this problem with. I would venture to say that you are having problems with either Game Boy Advance games or both the older Game Boy & Game Boy Color games, but not all games. These have 2 different processors in them, 1 GBA, the other for the older games.

You are going to need to replace the motherboard to solve your problem. You can find GBA SPs with broken screens and good motherboards on eBay for $15 or less including shipping. You will need a small phillips head screwdriver and a tri-wing screw driver to exchange out the motherboard. I would advise taking the one with the bad screen apart first. That way you will have a better idea of how to remove the motherboard without harming the video cable. The video cable is attached on the top side of the motherboard. When you go to remove the motherboard from the game system you can't move it too far or to fast before you detach the video cable or you can ruin the video cable. Good luck!