These are some of the performance related things I think about when developing games...
With your art keep in mind the power of 2 (32, 64, 128, 256, 512, 1024, 2048), as images are decompressed and often processed as power of 2 images, and if not already at power of 2 will use the next size up.
E.g. A 513x513 image will process as a 1024x1024 which uses 4 times as much as graphics RAM as 512x512
Anything below 256 is generally fine as they don't take up much memory, but above that you'll want to think carefully about sizes.
It's not as much of an issue with PCs and dedicated graphics cards, but is important on mobile.
https://www.scirra.com/blog/112/remembe ... our-memory
Use triggered events, functions and Timer behaviour instead of checking/setting things every tick. E.g. On Touched/Tap Object, instead of Is Touching Object
Frequently updated text, e.g. score/counter/timer, should use the SpriteFont object instead of the Text object, as the Text object lags a bit if updated every tick.
Turn off collisions for objects that don't need it e.g. background and GUI objects
Don't have a platformer object offscreen falling into eternity - I forgot to delete one once and got bad performance as it kept changing render cells every tick