I'd say when you need to slingshot the traffic levels will be busy enough that only 2 cars can enter the roundabout per gap, depending on how many lanes are used. At Gosforth Park, I'd only hold 5-6 cars for a few seconds longer than necessary, which judging by the number of people who slingshot wouldn't make much difference.

If everyone started slingshotting then there would be a problem, but 99% of drivers would happily sit in a queue. At the end of the day, I'd rather slingshot to stay on time and prevent queues forming onto the A1 roundabout than sitting like a lemon getting later and later.