When I make games they usually run too slowly, or worse run fine for me but impossibly slow for everyone else.
Here are a few tips to stop your games stopping...

1. Don't bother, just tell everyone to get better computers (not recomended)

2. If possible shrink the screen to 320x200 or a small size.

3. If your game doesn't scroll and you still want the game to be maximised without loosing speed, try this:

Insert a DISPLAY PROPERTIES object and at the start of the level tell it to go to the resolution of your game. (do NOT use 512x321, it always crashes for me).
If this is the same size as your game that's it!
If this is not the same size, go to the game options and set the window size to the size you chose before, select RESIZE TO FILL WINDOW but not MAXIMISE.

4. If your game runs fine for a while then slows down, insert a counter to count the no. of objects. If this counter goes way above what you thought it would, try this:

Filter all the CREATE OBJECTS events in the code editor.
o. Make sure there isn't an ALWAYS event that creates objects (unless there is supposed to be)

o. Look for an object that is being created lots of times but isn't moved or values set. Tell it to POSITION the object there and PASTE IMAGE INTO BACKGROUND AS OBSTICLE, then when you want to know if sompthing hits it use hits into BACKDROP.

o. Check for objects being created, doing nothing, then being deleted and remove them.

5. If you have a BIG active object but it has no animations and doesn't move, replace it with a backdrop. Backdrops are much smaller than active objects.

6. If you have a BIG active object that has animations or moves, but is mainly one colour, change the level colour to that and in the object replace all of that colour with transparent. (I use this one all the time)

Ok, I'm out of ideas.