How do I optimize my framerate with a lot of tiles ?

Hi.

Constant 60 FPS with this :

http://i.imgur.com/XcwEXhQ.png

Floating FPS (35-50) with this :

http://i.imgur.com/rxe8in5.png

Why is that ? Could it be the consequence of the different hitboxes from the diagonal tiles ? How could I sort of "fix" that framerate issue ?

I think I should add that my tilemap has a physic unmovable behavior. And moveables characters and stuff.

Might be possible to just make the tile map a solid instead of physics unmovable.

Is the background many sprites? One giant sprite? Tiled background?

yeah make it a tilemap instead.

I removed the physic behavior but that doesn't change anything. I also removed the background, no improvement either. :/