I know FJ offers one free withdrawal every week for their user and it may take few mins to hours to proceed your transaction no matter it is free or paid. And look at the most of the gambling sites they never mention instant withdrawals, they mention it ad high priority which means your transaction may be procceed faster but not immediately at that time when you wanted to withdraw.
FJ is not alone that allows one free withdrawal per week, NitrogenSports does that too. But I was talking about Sportsbet where I do not get charged anything whenever I withdraw my funds. Instant withdrawals are not allowed any more on those websites because the price of btc and eth is too high and even if they charge big, they cannot afford to send prioritized transactions and batch it up to earn big on those fees.
They can really utilize this network issues or bloatup for them to make even more money with those fees this is why it would really be done in batch.
Based up on experience with FJ, it does have instant withdrawals but giving out an option neither those transaction would be in low or high priority.
Gambling sites are almost having instant withdrawals nowadays.Only a few do goes having normal processing.