Whether 9S meant it or not, the implication that Strange can't keep up pisses him right the hell off. Star Guardian or not, he's a nightrunner and a magician. Of course he's okay to keep going, he will make himself okay to keep going, he's going to show all these assholes the true strength of English magic.

As such, the spell Strange casts is a bit stronger than expected. He finishes casting and gestures to one of the creatures. The gust of wind that Strange sends over towards an approaching void creature is strong enough to knock that thing off of it's feet and slam it back into another one. Petty spite is a great source of energy to get things done.

"I fought in the peninsula," Strange scoffs, putting some of that cocky bravado back in his voice. "Taking care of these things is easy compared to that!"