I timed it once mid afternoon and it took 28 minutes with no hold-ups at all so 15 minutes is clearly impossible in the extreme. Mind you, total time from aircraft steps to M25 was 41 minutes (hand baggage only) so I suppose that would compare quite well to LTN and STN overall.
41 minutes? Better than Luton if you get caught in a long immigration queue early or late at night. If no queues and you are being picked up or are in the short term car park maybe 25 minutes from Gate to M25 but probably closer to 30 minutes.
Around 15-20 minutes for biz jet passengers depending which FBO is used.