SDL2 - Emulator slow on Ubuntu/Mac OS X

I wrote a Space Invaders emulator in C++ using SDL2 only for creating the game window and for playing sounds (sdl2_mixer). On Windows the emulator works at 60 FPS (I can change this value to whatever I want and it works without any problem) but if I build it on Ubuntu or Mac OS X the game is unplayable (maybe 10% of the wanted FPS).

Is there an explanation for this?