Technically yes, because Zelle is just the frontend, but banks can settle between themselves via ACH if they don't have another arrangement or a RTP (real time payment) setup.
To answer the original question, because Zelle is ran by a consortium of banks, they all trust each other enough to release funds immediately and settle up overnight if need be.
Zelle isn't the ACH or RTP though, is what I'm saying. Zelle is the real-time notifications and accounting, but the reason every bank needs to have bespoke support is because it's each bank's job to figure out how to do the actual funds transfers in the background after Zelle payments go through.
Zelle payments, under certain circumstances, will be converted to ACH transfers vs RTP instant payments. Typically, this will occur with higher risk payments (unseasoned contacts or high value transfer amounts) or when the destination is an unenrolled participant in the Zelle ecosystem.
The past several transactions I've done with Zelle, it has taken 3-5 days for the money to arrive. This was with a contact I sent money to before using Zelle (my brother), and also a new contact. I'm surprised because my experience has always been Zelle was instant. Has something changed with Zelle? The bank I am using is Ally.
Nothing's changed with Zelle AFAIK, but the time it takes depends on the sending bank, the receiving bank, the amount of money, the history of both parties, arbitrary fraud metrics decided by both banks and EWS themselves and some other things. Sometimes they just decide that your transfer won't be instant.
I mistakenly opened a Bank of America account last year, but they had so many fraud protections on Zelle it was nearly impossible to use without constantly getting transfers held up for days and having to make phone calls to banking support to approve transactions and verify my identity, and it was such an absolute nightmare I will never ever bank with them again or recommend them to anybody. The worst part was absolutely the phone calls, I could never dismiss or approve anything online I always had to make a phone call during business hours.
Then I moved to US Bank and they support instant Zelle transfers, recurring Zelle transfers (which both my credit union and Capital One didn't support!), approving fraud alerts online and more, the only way I could be happier with them is if they had less security theater (like Mercury) but I'll be moving to Mercury anyway once they support Zelle.
