Thanks for all the input. All my current clients are in London and so I haven't needed to use LHR for a few months (for this relief much thanks). It sounds as though there may have been an improvement in security handling over the last month or two.
If Greg Ward has a target of 90% within 10 minutes then I would guess (a precise estimate would require a knowledge of the variance of security processing times) that the average queueing time would need to be around 5 minutes, which would be rather impressive compared to my prior experience.
There's always a danger in a reward system that's focused too closely on a single target. As 13Alpha rightly points out, what pax (at least, this one!) really want to know is how long it takes between walking in through the terminal doors and entering the departure lounge, and the security queueing time is just one part (albeit a significant one) of this process.