The delays have little to do with technology, and a lot to do with regulations and bureaucracy. Largely put in place to combat money laundering, fraud, terrorist financing, etc etc. Yes, there's some small banks that have horrible APIs and shoddy servers, but for most transfers the delays come down to things that if cryptocurrency were ever mainstreamed would be tacked onto it.

That is why it takes time to get approved on coinbase and other exchanges that try to follow the law.