Single Player Mode Extreme Lag

Singleplayer Mode is extremely laggy! I can't even walk. Anytime I loom around, walk, or just try to do anything it freezes and I have to move the mouse continually until it unfreezes. I thought it was just the generally loading in lag, but I've been trying to get it to work for like 2 days and it still just lags, glitches, and freezes.