The speed of transactions is dependent on the current blockchain congestion and mining fees you paid. For instance, if you paid a small transaction fee, then your transaction may take longer compared to the one who paid a larger fee.
Bitcoin Blockchain becomes congested very frequently, which means it will slow down the process of confirmations. This means it will take longer time to reach six confirmations, in a few cases it will remain as ‘Unconfirmed Transaction’.
To know how congested the Blockchain is, you can check the amount of ‘Unconfirmed Transactions’ on the Bitcoin network.