Under Scratch 2 a Wait [0] block will yield for one pass through the list of stacks that need to be executed. In Scratch 3 it appears to yield for some amount of time and the calculated frame rate goes very high as a result. The "Add a wait block to the motion loop" controls the use of a wait [0] block in the forever loop where the cat and bat are moved. When set to 0 there is no wait, when set to 1 it uses a wait [0] block. In Scratch 2, when set to one, the frame rate stays at 30. In Scratch 3 my laptop shows a value of > 50,000
http://beta.scratch.mit.edu/#259055091
