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.