Reducing lag is pretty easy. You just have to make sure you use transitions pretty often and try to keep the number of items between two transitions as low as possible. If you are planning a room with plenty of trees or an important army of enemies, you might as well make sure the room is the only thing between the said two transitions. In short, transitions are your best friends here. It also to be noted that disabled items seem to not cause lag: when using lots of trees, make sure to disable them for a smoother gameplay.
On the other hand, loading time is pretty hard to fix. It is based on the level's length and the number of items, and transitions don't reduce the loading time at all. A few items seems to make the loading time bigger: these are rotating blocks and moving platforms, as their specific settings take a longer time to load than a decoration item. To cut down the loading time, you might want to try to remove any useless rotating block or moving platform.
A large level with lots of tiles to load will also cause loading time to grow bigger, and may even cause lag where the tiling is complex. Make sure to delete all tiles that might be outside of the field of view of the player if you are trying to reduce loading time. At normal zoom, the player's field of view goes up to 4 tiles up, 5 tiles down, and 7 tiles left and right. Outside this range, as long as the player can't go further, any tile is useless (as long as you add a notice in your description to not zoom out).
These are about all my tips so far. I hope they have been useful. Of course, sometimes loading time can't be fixed because there is no tile to remove, no rotating blocks that is useless. The only last possibility is to make your level easier (or at least harder to actually die in) to avoid having to load the level more than twice.
StarEDIT: Derp, fixed bad grammar
Posted: August 28th, 2013, 3:08 pm