This is exactly the reason. The important follow-up question is: which creators?
Patreon supports two fundamentally different groups. One uses the platform for pure donations, where there are no exclusive perks to becoming a patron. This group doesn't care about faster payout because there's nothing riding on it.
The other group uses Patreon as a paywall platform. They care a lot about payout speed, to prevent people from joining quickly to gain access to the paywalled content, then canceling their payment before start of the next month.
The paywall group is where the money is, because Patreon's model doesn't provide the mutual assurance necessary to convince people to fund public (unrestricted, infinitely-reproducible) goods. And so, inevitably, the platform will cater to the paywall group.