> why oh why does it take 3 days (in the US) to transfer funds between my own accounts in two different banks?
Wire transfers are instantaneous [1]. They cost pennies a transaction, a fee banks often cover if you have more than ~$15,000 with them. For smaller transfers, real-time P2P was recently rolled out across most American banks [2]. This was a result of competition from Venmo, which one could think of as a lightning network over ACH [3]. (The latter being Fedwire's net settled and thus slower but cheaper cousin.)