Bit of a mixed bag, when the demo loaded I hung around 40-45 FPS for about 30 seconds, before hitting a steady 59, never hitting 60.
When I hit fullscreen, the game instantly hit and stayed at 60, dropping to 57 infrequently.
The real change came when I went into video settings and changed the resolution from 1366x768 to auto, which I assume uses the native resolution (in my case 1920x1080).
After this the game tanked - dropped straight to 30 FPS, then up to 49, 56, 50, before settling at 46. The strange thing is that 46 FPS at the auto resolution is far worse than 46 FPS at 1336x768.
Specs - AMD A8 6600K APU w/ RADEON HD8570D. APUs aren't super common, but it shouldn't be having trouble with a 2D game.
EDIT//
Fullscreen adds a solid 6 FPS to my auto FPS, jumping to 54 - it's pretty steady too, and definitely playable.