Play your game in "debug" mode to see where things are slow:
1. Create a group called "test group" and move things into and out of this group. Narrow it down to a few event sheets that are the biggest resource hogs, and try to optimize from there.
2. Put a text object called "fpstext", or something similar, in the upper left hand corner of the screen. Every tick, set fpstext's text to the following:
round(fps)&newline&" FPS"&newline&round(cpuutilisation)&"% CPU"[/code:2z1y921z]
With this in place, play the game on mobile. If fps is low, but CPU is also low, it means you are limited by the GPU and need to use smaller textures to reduce the number of time pixels are being told to redraw each frame. If fps is low and CPU is high, then you need to optimize your events because your mobile CPU can't keep up with your desktop.