Per your multiworld troubles...

Ive copied this from my other post in worlds plugin. NOT SPAM just thought it belonged here.

http://dev.bukkit.org/server-mods/multiworld-v-2-0/

http://dev.bukkit.org/server-mods/transporter/

http://dev.bukkit.org/server-mods/skylandsplus/

http://dev.bukkit.org/server-mods/magiccarpet/



Here is a collection of plugins I use that may work for you. Multiworld: Create separate worlds which players can travel to. There are 8 world types: Solar, Planetiod, Flat, normal, the end, the nether, Pixel Art, and it can use another plugin to generate a world, like skylands. And John, to answer your question, It won't lag at all, because the other worlds don't load until a player occupies them, and even then it's not very taxing. Also, it's designed to work well with other plugins; the coding is very clean. However, excersize caution with planetary worlds, exploring too far will lag the living sack out of you.

Skylands+: Creates a world made of floating islands in the aether with biomes and everything this specific world gen will tend to lag a little too if you do something stupid.

MagicCarpet: This is not a world gen nor a multiworld utility, but it will allow players to traverse through skylands and planetary worlds. It spawns a glass carpet underneath of you that moves with you. Jumping raises altitude, and sneaking lowers it. You can configure what its made out of in the config. Players can also create custom carpets.

Transporter: Allows you to create cross-server portals, and walk-through portals to other worlds. This requires some networking know-how to get working for the server-to-server transport. it also requires ClientPatcher.