With every Bitcoin transaction, the sender has to implement a fee for the miners.
This fee is normally automatically set by your Bitcoin wallet to ensure a quick completion of the transaction.
When the Bitcoin network is exceptionally busy, the miners will serve the transactions with a higher fee first and the transactions with a lower fee will have to wait before they get their 3 confirmations.
To give your transactions the right fee for a quick completion, we recommend you check https://live.blockcypher.com/btc/ and if necessary manually change the fee your wallet uses with your transactions.
Please remember that this is not a guarantee for a quick completion but a recommendation we give our customers.