Indeed I had in LB a 5 sec delay on all my emus so I changed them to zero. Then in BB I checked my Startup settings and it already had 5 sec. When I launch a game, same issue of the menu splashing for a few seconds after Startup screen, before MAME launches. Also, when I return to LB, all the emus reverted to a 5 sec delay.
It appears the LB & BB Startup delays are one in the same. You may have been thinking the emu setting was a delay before it launches, and the BB was how long the Startup screen appears, which would be great. Regardless, I'm still in the same place as I started. I assume you don't have this issue?