I have had more than two occasions where my withdrawal of bitcoin from an exchange has delayed for more than 4 hours even after authorising and confirming the transaction through my e-mail. I don't know if am the only person that this particular problem has happened to. I want to know the possible cause of the delay in transaction during withdrawals.
Transactions on the Bitcoin network itself aren't controlled or confirmed by BitPay, but by the bitcoin miners which group transactions into "blocks" and add those blocks to the Bitcoin "blockchain" – the shared historical record of all transactions. When a transaction has been added to a block six blocks ago, it's considered a done deal.
Currently, bitcoin network traffic is unusually high due to increasing demand for transactions per block. Block sizes are limited, so this means that transactions which exceed the capacity for a block get stuck in a queue for confirmation by bitcoin miners. This queue of unconfirmed transactions is called the bitcoin mempool.