Detec touches outside update

I'm developing a 3D game for mobile in Unity. In order to move the game character, I'm using GUI textures (as buttons) and then a touch detection in the Update function. The problem is that when I test the game in 200$ mobiles, the character moves very slow. I think that's because the code inside Update gets called once per frame, so if fps decreases, the code doesn't get called as it should. Is there any way to detect touches (1 and two at the same time) outside the Update function? Before I used OnGui, but as I said, I need to detect if two buttons are being touched simultaneously so I couldn't.