Wallace tree multiplication rules
The pairs of 3 are sent to a carry-save adder, which takes a sum of 3 inputs and reduce it to a sum of 2 inputs (x + y + x => c + s). A carry-save adder's propagation time is constant, while traditional a 2 input adder's propagation time is proportional the the width of the operands. You can also use a half-adder, which performs the reduction (a + b => 2*c + s) in constant time.

For the multiplier to be as fast as possible, you want the layers to use only carry-save and half-adders, as well as have the final 2 input addition to have the lowest width possible. Like this, all your layers performs in constant time, and the final one has the lowest propagation time possible.

You should find that if you reduce the pairs of 2 in the first layers with half-adders, you won't save any layers or reduce the width of the final addition, thus the multiplication won't be faster.