I would assume that the delay comes from the fact that the OS has to keep skipping your if statements. 15 is quite a bit of skipping.

I would suggest that instead of your if then, you make one big string and use sub(. Each room is 16*8= 128. So you would make a string, "roomA[128]roomB[128]roomC[128]...". All one after the other. Then to access them:

sub(str0,(room)*128,128->Str1

Will this work for you? Because this should be near instant once you have your string stored. Or if you can't afford to give up a string for this, you could use:

sub("roomA[128]roomB[128]...",(room)*128,128.   

It will be a little slower, but it doesn't kill any variables.

Ya, I would agree with Penguin on that one. Just have them lined up in an order that makes sense for when you go room to room then just display the whole thing at once. Though are you doing this on the homescreen or graphscreen? You said Dual Layers so I'm assuming graphscreen which means you can't display the whole map at once (unless I'm mistaken).

I just saw this, so something for people who don't have rooms (or something similar) the same size and yet want to have a bunch of rooms in a string:  it helps if you have someway to tell the calculator "this is how big the first room is, this is how big the second room is," etc. so that you can have quicker searching for the calculator.  If you have a room that is 150 bytes in length, another 200 bytes, and a 3rd 100, and if your calculator has to search through the first 350 bytes to find out where the 3rd room begins, that is going to be terribly slow.

I have a function that takes a sprite--and its mask--and draws it out.  The mask data occurs immediately after the sprite data.  However, instead of having the calculator compute where the mask is, I just tell it "This is where the mask is."  It saves quite a bit of time.

Ya, I was gonna suggest that since the further you get into a string the slower it goes. If you wanted to make the make drawing as even as possible you could flip the strings around (so one starts with room one and then the other ends with room one). I don't know if that would actually work but in theory it does Just a thought.

I believe they were displayed the same way you are doing yours I believe each layer was a single string, and they were displayed in a similar way