In my case (janpier linked the post) eventually it was nothing graphics related. manipulating graphics size, resolutions, color palettes and stuff had no effect on performance.
I got to QUADRUPLE (x4) the FPS by optimizing events and code, and that was still a little too slow for old devices, so I aimed at 30fps for mobile and it's still a little slow but bearable for old devices.
You can see what I've done there to optimize the code - nest events as much as possible, disabling events that are not needed in runtime, user built in movements instead of manually setting X and Y of objects wherever possible.
I see on Google Play many platform games with the same complexity that run silky smooth 60fps on my old test devices and I wonder why. Is it the exporter, is it the engine, is it us - I don't know.