I already explained this in literally the last reply, please don't regurgitate already-explained things.
We don't know that's the reason, and it's unlikely given that even when Langris first attacked Asta, it was keeping up with him
Even if Langris was already in Rage Power, it seems like he can simply boost himself even further with it. Further supporting this is Langris saying "I can get even stronger than this."
So basically, whatever this Langris is, he's relative to Asta (1.38c), then he boosts himself even further (if he's in base, he goes Rage Mode; if he's already in Rage Mode, he just boosts himself even further) and jumps to 2x the speed (2.76c), which Black Asta scales to for blitzing this Langris.