Why does only one world in minecraft lag so much?

There's a lot more going on in a rainforest biome than almost any other, and that makes it run slower. Try turning everything down in the video settings; graphics, smooth lighting, render distance, everything. It should help a lot. Also, turn down the weather volume; I find that helps depending on the computer.

Areas in your Minecraft world(s) can cause increase demand in processing power, which does tend to cause your framerate to plummet as a result. Here's a few things that can cause such lag.

* Random Generation of Terrain -- Whenever you explore OUTSIDE the known parts, your system has to work a bit harder to crank out the new chunks to fit your world.

* Massive Running Redstone circuitry -- Generally, the larger your Redstone circuitry for a project, the more processing power is required to keep it running properly.

* Massive amount of mobs in an area -- As annoying as it sounds, if you cram a lot of mobs in an area (like livestock or villagers), it can cause framerate issues that there's just a lot of stuff moving at once that the game has to keep track of.

* Complex effects -- All that "eye candy" takes a lot of processing power & Java doesn't have any shortcuts to push more of that on the graphics card.

Besides lowering your video settings to reduce demand on your system, try eliminating the potential causes. The less dynamic an area is, the less processing demand there's to update it.