How propagation delay of synchronous counter is less than asynchronous counter?
In every Flip-Flop there is the clock-to-Q delay.  

In a synchronous counter all the outputs are updated with a single clock to Q delay.

In an a-synchronous counter the first FF updates after one clock to Q delay. The second one can only change after the first one has changed so it updated in two clock to Q delays. The third one needs three clock to Q delays etc.