Do you mean lag from when you play online or frame rate issues when your offline?
If your lagging you should try these methods.
1. get better net speed
2. Check to make sure your wires, router are working properly.
3. Make sure no other programs are using the internet.
If you are experiencing frame rate issues while offline.
1. Download optifine the program can help lower frame rate problems.
2. If you are using a texture pack it maybe to much for your pc to handle.
3. Close programs you do not need up.
4. Try to stay away from the nether, and snow biomes. Those 2 biomes in general tend to cause most slowdown issues out of the other biomes.
5. You might have to many mods installed and may need to get rid of some, depending on what mods you are using.
Those are the only solutions I can think of at this time, if I think of others I will let you know.