Asta E does bounce damage that hits up to 5 enemies. For each enemy hit, Asta increases all party members' atk by 10%. Effect lasts 15s. E cd is 10s. This alone can make Asta broken in aoe situations, a kind of Bennett but with E. But in single target scenarios, it is just 10% atk. So it is balanced.
Now Genshin speed is different from the speed in the turn-based HSR. Instead of buffing speed/advancing action, in Genshin, Asta Q can reduce all party members' E and Q cd by 5s. Or reduce all cd by 20%. And the effect can last 25s.
There are not too many abilities in the game that reduce the team's whole cd. There is anemo resonance (5% cd reduction) and Raiden c6. But Asta can be a unique kind of support in Genshin, with the role of cd reducer and atk buffer.