How big is your file : 10 426px x 7 802px ? That's not so bad, but with 1.5 GB you must have a lot of layers! If you continue like this you will hit a wall. You should try reducing the nuber of layers or deleting/merge unnecessary overlaping layers. You could also try having multiple files containing differents layer and only merge them for the final version. That way you don't have to endure to work with a massive 1.5 GB file, where just using the pencil makes the computer lag.

Just saying because you map is very detailed and is looking good but you need to keep an eye on it's size.