Actually would be interested in some feedback on that as the only other time I've transited at LHR was on Christmas day last year and it was deserted...
Very much depends on the time of day, I think, and even then...
Most times I've come off the BOM flight between 6 am & 7 am and the security queue at the Connection Centre is fairly small, usually no more than 5 mins. Couple of times, though, it was absolutely heaving and I managed to divert down the Fast Track after giving a winning smile to the Guardian Dragon so I could catch my flight to EDI.
On the way back I've noticed huge queues around 8:30 to 9 am when I come through the side door from T1.
I think it builds up once the European flights start to come in.