BHX no longer has the infrastructure to provide this single passenger throughput. Consequently I question its "efficiency".
Again... there has to be a business case. Infrastructure is expensive and the airport needs a worthwhile return on what it invests.
For example, no airport is going to invest in say 5 new security lanes that may only be needed for an hour during peak periods (atleast not unless your airlines are willing to stump up for the costs). The airport is never going to get any kind of financially viable return on that. Instead, they have to ride those kind of capacity issues an that's done by building greater efficiency into their operation.
Granted any "efficiency" may not always seem that obvious to the customer, but it's about adding longevity to existing infrastructure to hold off from any costly investment until it's absolutely necessary. Remember the airport is not a charity, it has shareholders to please and has to succeed in a ruthlessly competitive market.