With payments received for tangible goods Paypal would request valid tracking information confirming the item has been delivered to the buyer. As they can validate the tracking information online, once it is delivered they can release the funds. IF you have received the funds for a service or no tracking number is applicable and Paypal would be unable to release these funds any sooner than the 21 days.
If you mark the transaction as processed then the buyer may have the option on their account to mark the transaction as delivery/satisfied. If the buyer confirms this by clicking on the "Confirm Receipt" button, the funds may be released to you sooner. This is not always guaranteed. If not then sadly its 21 days.
Advice is voluntary. Kudos / Solution appreciated.