That’s my experience up to Heresy (difficulty 4) with random players.
I know some might prefer shorter missions to play more missions in a limited time frame. It’s probably always tricky for a game to offer what’s best for most of us.
20 to 30 minutes gives time to get our bearings with random players and find some synergy (when possible and without the occasional headless chickens), push to higher intensities and the finale when things get hectic and extra fun/hard.
Great pacing imho.