I have to say that security delays have significantly reduced over the previous six months. I've been doing a weekly flight (sometimes North terminal, sometimes South) from LGW over that period and since about November he number of security staff and the number of scanners operating seems to have about doubled.
I used to check in on line (hand luggage only) but still leave two good hours from arriving at the station to boarding, now I'm very comfortable with just an hour and a half. Even that is generally over the top, but I just hate the feeling of leaving it to the last minute. As an example a 10am flight last week from the North terminal, I had a choice of three scanners, not one single person in front of me at any of them.
It will be interesting to see if this is maintained as we start moving into holiday periods,
vslf