Happy xmas !
Generally speaking, yes the withdrawals are instant. When you submit your withdrawal, it goes into the processing queue.
The payment processor on the servers that checks for new withdrawals requests each second and sends the details to blockchain via their API. Sometimes there is a delay in communicating or if the API is not responding but the server keeps trying to send until we receive the transaction details back from blockchain.
If yourself or anyone does experience an unusal delay, please let us know via the feeback tab along with your username so we can chase it up.
cheers,
diablo