The game looks great. I like the graphics, though it's a little odd that the mob is drawn top-down and the walls aren't. I'm a sucker for simple graphics and nice colors with black outlines =)
As I'm sure you're aware, it took a while to start up. Is it really just because there are so many maps and objects?
In some projects with autojoining turfs I'd use the turf's constructor to update its icon_state. As I added maps the game started more and more slowly. I didn't suspect this at first because with a single decently-sized map it'd cause no visible delay, but eventually it added up. If there's any processing like that that could be delayed instead of being done in the turf's New() proc that might make a bit of a difference.
Also, the game is very choppy when I move. The clouds move smoothly as long as I'm standing still but as soon as I move it gets very choppy. I've had problems like this when using screen objects with pixel movement. The BYOND staff has told me that they've fixed this problem but it's certainly still around. I'm uploading a video, I'll add the link here when it's done.
CPU usage (while standing still) was around 30-40% outside and dropped to 15-20% inside the first cave. I'm not sure what the difference was (were the clouds really that much?). Also, it's on a dual-core CPU so the usage numbers have a max of 50%.
Edit: Here's the video: