The phrase "impressive composure" means someone staying calm, collected, and composed in a way that is very impressive and worthy of admiration. Full definition
The 2017 Mercedes - Benz GLS handled the tight and winding roads in Austria with impressive composure for a vehicle this big and heavy. (cnet.com)
Push things a little harder and the car still goes about its business with impressive composure, while my test car's optional carbon - ceramic brakes slow things down in rapid fashion. (cnet.com)
His second goal that put the result beyond doubt showed impressive composure again, as he moved the ball from right to left before finishing coolly. (eplindex.com)