Zelle is not always instant. There are two ways for a Zelle transaction to clear. The fast one goes through the Visa network, the same network for credit cards and debit cards. The slow ones goes through ACH, which takes several days. It is generally not possible for two recipients to know which network Zelle will use beforehand.

ADDENDUM: The official marketing page for Zelle at https://www.zellepay.com/faq/how-long-does-it-take-receive-m... has this to say about transaction speed:

> Money sent with Zelle® is typically available to an enrolled recipient within minutes.

> If it has been more than three days, we recommend confirming that you have fully enrolled your Zelle® profile, and that you entered the correct email address or U.S. mobile number and provided this to the sender.

Note the weasel word "typically" in the first sentence. If all Zelle transactions went through Visa, all such transactions would be instant and they wouldn't have the need to add the word "typically" in the first place.

The second paragraph asks you to wait three days. That's basically the Zelle fallback path when it uses ACH under the hood.

ADDENDUM 2: This HN conversation really piqued my interest and did more research. So apparently Zelle now uses one of three different networks. According to https://www.americanbanker.com/payments/news/how-the-clearin... :

> The Zelle P2P service currently uses two separate rails to settle payments, depending on the particular transaction, including the ACH network and the two debit card networks: Visa Direct and Mastercard Send. By adding the RTP network, Early Warning will be able to reduce its reliance on ACH, which can at times take days to settle and is operated only in batch mode during business hours.