Then the problem wouldn't be going cashless, it would be the UX. There is absolutely no reason to have a tip screen at a football game. No need to push a button to confirm payment either. Very easy to just have the amount display, tap to pay, done, like most card terminals out there.
Also, even when cash was allowed, I'd almost guarantee you most were choosing card anyway, so going cashless wouldn't have had a major impact on wait times anyway. Only 10% or so of payments in the USA are cash.
Also also, very very very rarely do cash people have the exact change ready and out of their pocket and ready to hand over to just "take their items." Usually it's FAR slower than the card method while you wait for them to pull out there wallet/purse, count out the bills they need, then wait for the cashier to count back the change.
So IF wait times really are longer, I think the culprit is something other than cash/no cash.