Our B Meet lineup this year was 2 D1s, D2, D3, and D7. In my experience, the top-division teams cover the full spectrum from chill to intense. Langley and Hamlet (who we schedule most years) are great and we always have fun B Meets with them, home or away. The D1 elite -- the pools that are there every single year -- are much more serious about everything. But both of our D1 B Meets this year were fine.
On the 7:40 finish -- we started at 5:30, which helped, and no IM. But there were still multiple 5 heat races for the little kids. The biggest efficiency was an extra referee who could handle the stroke/turn DQ discussions without a big delay to the start of the next heat. That one added efficiency likely saved 30 minutes by itself. (I also think that all of the stroke/turn judges -- ours and theirs -- took a fairly relaxed approach to DQs . . . perhaps out of necessity given that each judge had to cover 4 lanes instead of the usual 3.)
Our team is around 150, excluding minis. Although our our typical B Meet attendance is likely closer to half that number.