SWIFT is generally reserved for international transfers. In the United States domestic bank transfers use ACH which involves FTPing fixed-width-column EDI files to and from the Federal Reserve every night. ZenPayroll had a really good series on how it all works a few months ago:

Part 1: http://engineering.zenpayroll.com/how-ach-works-a-developer-... (HN: https://news.ycombinator.com/item?id=7636066)

Part 2: http://engineering.zenpayroll.com/how-ach-works-a-developer-... (HN: https://news.ycombinator.com/item?id=7740967)

Part 3: http://engineering.zenpayroll.com/how-ach-works-a-developer-... (HN: https://news.ycombinator.com/item?id=8007838)

There’s a good 5 part series on this from 2014: https://news.ycombinator.com/item?id=7636066. Fascinating how slow and brittle the system is. Even more so that it’s not been upgraded to something real-time (see:Europe).

There's an interesting series of blog posts on the topic: https://engineering.gusto.com/how-ach-works-a-developer-pers...

And the accompanying hackernews thread - https://news.ycombinator.com/item?id=7636066

But basically it seems like ACH is slow because the federal reserve runs once a day batches.