Нет, быстрым он не будет. Конечно относительно быстро чем если бы они вообще ничего не делали со сложностью, но на практике это все довольно долго.

How the difficulty drop is triggered?

The difficulty drop is triggered based on six blocks taking longer than 12 hours to mine. The MTP (median time past) of the last block is compared to the MTP six blocks ago. If they are more than 12 hours apart, then the difficulty drops by 20%.

What this means for the hardfork?

After the hardfork Bitcoin Cash's hashrate will drop from the combined pre-fork Bitcoin hashrate. Exactly how much hashrate it will get is open to speculation. If it gets less than 8.3% then the difficulty drop will be triggered. (The drop could still be triggered with more than that based on variance.)

The difficulty drop does not happen immediately. The drop won't happen until at least six blocks after the first slow block. At least the first six blocks after the fork will be at full difficulty.