This might be of some use (from the brussels airport site):

http://www.brusselsairport.be/en/air...nsferGatesBtoA

I have flown IAD BRU many times, though only once with a transfer. The only delays I have ever had was waiting for my bicycle box once at the odd-size luggage area. I flew with only a carryon last time and was through passport control and customs about 8 minutes after getting off the plane. 6 of that was the long walk down the hallway.

There also aren't too many other flights around at that time. I feel like most people in the baggage claim area are on their way out already by the time the United flight lands, so I would guess most transfer pax are already through transfer security at that point.