A MasterCard deposit casino transaction will nearly always be handled instantly. Casinos know players would rather not wait to play, so they make sure deposits are instant in 99% of cases.
When it comes to withdrawals, though, MasterCard might take a little while. That’s because as well as the casino, the bank has to process your withdrawal.
The casino also gets charged a higher fee for using cards. So, they usually encourage people to use other options by putting card transfers lower down the processing priority queue. But players still use them because they’re convenient!