Then that probably explains your lag. By default, /fpslimit is set to 20, which means that if you don't change it (change it by typing /fpslimit 20-100), you'll get very choppy game play. Lower numbers are choppier and higher numbers are smoother, but if it's too high and your PC can't handle that much FPS, you'll get "fps lag", which happens randomly, like when loading lots of textures.