You can also lower the interval at which MC auto saves (with Optifine). By default MC saves every second or 2 or something crazy like that, on a system with a hard drive that isn't blazing fast that can cause a lot of lag. Heck, my HDD is fast and even I notice a serious reduction in lag when I lower the save interval to once every 3 minutes (which means I'll lose 3 minutes of work in a worst case scenario, which isn't a big deal for massive lag reduction). Especially if your world is big this can probably help a lot.
Also, for some settings you might want to restart MC, so if you're messing around with them restarting MC to make sure they're properly activated is a good idea.