Ooof you know, I had worked before on fixing some issues with chunks (10x10 sections of tiles) not unloading properly, but forgot to get that merged in. It also means that the more you explore, all those tiles just stick around (along with the weather particle effects etc)... which I'm sure slows stuff down... will try and get that fixed this weekend, and hopefully it'll help with some of the slow-ness issues.
Thanks!