When you start an SSP world? No other worlds should be loaded until you create and/or enter them.
I had to install Optifine. Thankfully it worked for me this time. I think the problem is Extra Biomes XL's huge trees. My lag was most terrible around such biomes before I installed Optifine. You might want to try it, though it's a bit of a toss-up whether it helps or not.
