I had an idea for an optimisation:

The behaviour of forceLoadRadius could be modified to only keep the chunks loaded around portal pairs which are connected to a world with at least one online player.

Makes for a sucky experience if you only have one player online. I'm not sure what the behavior of bukkit is wrt. unloading worlds when there aren't any players in it anyway.

Feel free to close this if you don't think it's needed.