We've had a few reports of delayed order processing that may indicate some sort of issue PayPal's end delaying their ability to send IPN (Instant Payment Notifications). We depend on IPN to know when an order has been paid for, so we can process the order accordingly. We've opened a ticket with PayPal to bring the problem to their attention if they're not already working on it.
Affected orders we've seen do appear to be getting processed within an hour after the buyer completed checkout, so we'd advise just telling buyers to be patient as their order will be processed as PayPal catches up with their backlogged payment notifications. You can also check the IPN status of any particular transaction as we instruct under Part C, Step 2 here: