Chunk loading on a fully generated map is problematic. Included is a screenshot displaying the severity of the lag spikes caused by chunk loading.

I don't think the graphics are to blame, because when I start getting lag spikes like this and go to the video settings menu, the interface lags too.

Just now, I was going somewhere on a fully generated SSP map and the chunks stopped loading altogether. So I left the world and came back. The game got stuck on the dirt loading screen. I was in full screen mode and couldn't do anything. I waited 2 minutes with no change. I couldn't bring up the force quit dialogue or tab out. I had to reboot. This pretty much makes the game unplayable.