Due to the number of mods that the Feed the Beast packs have, there is an increased amount of lag associated with using this pack. Mods such as Traincraft, are still in beta stages and are unstable. Other mods such as ExtraBiomesXL and Mystcraft have massive amounts of terrain generation. This creates added lag in the game, and results in sometimes unplayable worlds. The best solution to this is to set your computer and game settings to the most optimal FPS settings you can, or remove mods that greatly contribute to lag. This seems to result in different set-ups for many users of this modpack. You may need to change your server or computer performance settings to compensate for any changes that have occurred.\
Make sure you are not running any extra programs in the background (web browsers, streaming programs) as well.
