All cryptocurrency transactions must be confirmed on the blockchain before credited to your balance. This is a completely automatic process and therefore there is no way to speed this up, as it is determined by the network.
Stake.us requires a certain number of confirmations on the network depending on the coin you choose, and this information can be seen via Transactions > Purchases.
The time this process takes is fully dependent on the state of the network as well as the fee used on the transaction (i.e. setting a higher fee will typically result in faster confirmations). Generally if the network is congested or under heavy demand then you can expect transactions to be slower than usual to confirm.
Please note: If you set your fee too low, your transaction can expire and your funds may be refunded back to the address/wallet you sent them from after a period of time. This is a rare case and not something we can control.