Having them faster means they'll take more turns for every turn of the CB, hence they should do more damage. But the idea with counterattack (or unkillable) teams is that you don't make all champs as fast as possible, but you give them such a speed that they all remain in sync - that they keep having counterattack buffs on at the time of the CB's two AOEs, for every turn the CB takes. If you don't speed tune, they will go out of sync, still counterattacking sometimes, but sometimes also missing out on counterattack opportunities because they have it up while the CB is doing his single-target stun on another champ, then they don't have it anymore when he hits the next AOE. Moreover, it will make it harder to arrange for each stun to hit Skullcrusher at the time when he's got his unkillable up, which may mean champions dying earlier than they should.