Anyone with a reasonable amount of practice can un-holster and fire their weapon is a surprisingly short period of time.
"It was "successful" because it was unexpected. "
As opposed to the expected random shooter. Again, I'm quite sure those security guards in the DC Jewish museum weren't expecting to shoot a man dead when they went to work that day, but that is why you train.
Despite your protestations, this man operated in an environment where he knew he wouldn't be challenged because law-abiding people were all unarmed. He was the de facto apex predator today. Had he been seeking victims in a environment full of people who could shoot back, he would have been stopped, even if shooting from a moving vehicle.
Gun owners are more prevalent in the countryside in Britain than in the towns and cities. He didn’t choose the most target rich enviroment.
Plus the DC museum incident you mentioned was perpetrated by an octogenarian whose sight and reflexes weren’t what they once were, and he’d already shot and killed one security guard by the time the other shot him....
Nobody expects a random shooter. Thats kind of the point.