UPDATED FOR GMS v2.3+
This script demonstrates how you can cut the number of objects in a room dramatically by "merging" them together to form larger ones. This is an ideal script for top down games and platform games, as it will permit more instances to be placed in a room, since it can lower your instance count by over 50% (as high as 80% in some cases).
Note that the scripts work by changing the sprite index, so you are advised to use TILES to draw the walls/platforms, then make the objects for collisions invisible and optimise them with this script.
You can find a companion script to add tiles to a room automatically based on the instances placed (also written by me) from the Marketplace here.