If written to her true specifications, Monica can rock with Thanos...at least hold out long enough until the rest of the team shows up. More to the point, not every villain is Thanos' level.
My hunch is that it's just a matter of time before some writer establishes that the reason why it appears that Monica arrives after everyone else is because the human eye can't really spot her at her actual flight speed. What we are seeing is something akin to a residual image. Monica is far ahead of the point in space/time where we see her.
It's either that, or she's doing the prudent leader thing and hanging back to better assess the full situation. Nah, forget I said that. She can assess the entire situation and local environs at the speed of light. She would already know what's going on before any of her teammates arrived.