FPS and frame drops

I have 50 FPS and the game drops them to 20FPS if I start moving in the latest version (1.8). I think there is a problem withe the GTX 680, because a good friend of mine has something around 300 FPS with his GTX 660.

I 'm using a MacPro and I'm working with iOS X 10.9.4, he is using Windows, but I don't think that's the problem here.

I hope is is going to get fixed pretty soon, 50FPS is not the main problem, but those frame drops sucks D:
Thanks Mojang, your are doing a great jop 🙂