Actually, the game wasn't lagging at all ! I used "printfps" (project settings > debug > settings > print fps) and it showed me the game was running at 60 fps. My animations are supposed to run at 12.5 fps, so it was way fast enough.
Then I saw that I could speed up my animation speed :
animationplayer sprite's settings > playback options > speed
So, the fps shown in the animated sprites' SpriteFrames is not used at all ?
In the animationplayer's sprite settings, I can speed up or speed down my animations, but how many fps is speed 1 ?
It's quite confusing, really... :/
(PS: I'm an animator, not a programmer)