I have been having difficulty playing my world recently due to all the stuff I have going on. Even with optifine loaded (utra, default config), it has been a bit laggy of late. With the latest update to 3.1.1, I again had to add in optifine. This time, I enabled the multi-core chunk loading and disabled thread optimization with my 3d settings as recommended. Wow! This is so much better on my machine. It is very nice and smooth, without the lag I have been experiencing. There is a slight (less than 1/2 second) delay while placing blocks and interacting with tiles (graphical update only), but it works so much better than before.