@dionysius: I take more than 100 flights a year. I know how long it takes to efficiently board an aircraft, even after making allowance for passengers blocking the aisle. 30 minutes is far too long for a 735, even by Servisair's standards.
If the pressure is to achieve on time departures, why don't the gate agents actually start boarding earlier? I know that there are times when a late inbound will result in a delay in boarding, but the ground staff at BHX always seem to delay the start of boarding.
@Daza: I am afraid that the wanderers and stragglers are likely to still delay things as they seem incapable of looking at monitors or hearing calls for them over the PA. There is a very good chance they will be in the departure lounge bars. In any event, any missing pax will only be identified once everyone at the gate has boarded. All the more reason for starting boarding early.