I deposited a large check once — nothing that would get the attention of the IRS or anything, other than that I’m normally poor — to a regular national bank. They held it for about two weeks to “verify” it, aka invest and profit off it, so I immediately went over to the local credit union (who doesn’t do shit like that) and opened an account there. Got a good chunk of it transferred with little-to-no fuss and closed out my bank account with a middle finger.

I can assure you banks absolutely do have the ability to send your money back instantly.

USAA does this for certain refunds, but they take on a bit of risk to do so. They make the funds available to spend before they've actually been settled

In Europe there's a system called SEPA, which is a network that allows instant transfer of funds. Right now, by law, banks are required to allow consumers to use SEPA at the same conditions as the regular old bank transfer, which means I could send you money right now for free and you could receive it instantly anywhere in the EU.

The technology exists. Banks just have no incentive on implementing it

I worked in retail for 15 years, and this came up all the time. Yes, your money has been “accessed” in your account, but if I run my journal and show it to you, you would see we don’t actually have it yet.

Wouldn't this create inflation... all that money "in transit?"

I think you're missing a point. Person A spends X dollars at person B's store. Person A immediately sees X dollars reduced from their bank account, and it shows as pending. Person B does NOT immediately see X dollars added to their account. For some duration, that money is just on hold while the bank verifies and processes the payment. Meanwhile Person A gets whatever they bought for X dollars on the promise and expectation that the bank will soon pass along that payment. Person B may not actually retrieve that money for several days, though.

Now Person A comes back for a refund, so Person B submits that refund back to them. The exact same process happens in reverse with the bank. Depending on the company, there may be an extra internal accounting/fraud review process for the business before the refund is actually submitted to the bank, which adds more time. But whether it does or doesn't, the time it takes for that money to transfer between accounts is entirely out of the business' control.

My family owns and run multiple retail fireworks stores, and I work in sales data for a major retail chain. I have personally handled many times when someone got overcharged for their transaction, like if an item got scanned twice by mistake. We immediately run a refund for the over charge in such cases. It still takes days to hit their account again. It has nothing to do with us or our processor how long that takes. If you have a problem with it, talk to your bank. They are choosing not to front you the money we refunded. It is already gone from our account, and is sitting in pending state.

But it’s the same thing. The banks just front the money essentially. The actual transaction doesn’t clear until later.