While Angry Birds is a simple game, the rigid body physics used aren't that easy to implement for a slow processor. You are essentially dealing with a set of rectangular 2D shapes that can be rotated through any angle and require reasonably precise collision detection. Each frame, you need to calculate the forces acting on each part, move them all accordingly by some small time delta and recompute their new positions. Any colliding bodies need to be then solved, which will result in a change in the forces acting on each of the bodies involved. Obviously, there are damping effects to apply which mean you can stop calculating the position of any body that has reached a reasonably steady state (until it is hit again), but it's all rather non-trivial. A lot of 2D vector arithmetic and normalization.
And all that comes before you have to render the frame, which would at least require a lot of rotated versions of sprites if you weren't going for a straight 2D texture mapper.