Read the tutorial Physics in Construct 2: the basics.
b]Note: Physics objects only interact with other physics objects. Adding the Solid behavior to the floor has no effect on physics objects. They ignore anything which does not also have the physics behavior. Physics objects will pass right through 'Solid' objects, unless you give them the Physics behavior and set them to immovable.
Physics should perform OK with up to 20-30 objects, enough for an Angry Birds type game. Are you sure Physics is causing the slowdown itself? Mobiles have pretty weak hardware, and simply drawing the screen can cause most of the slowdown. If you remove Physics entirely, does the game run a lot faster? If not, you must conclude that Physics isn't causing it to run slow.