The Finance Buff blog has a good review
of ACH transfers and why they sometimes take so long. I know some are suspicious that banks may be making money off the float when the transfer takes several days with a couple of days of missing interest. This blog post doesn't go into those details, but it does mention some possible reasons for the delay such as risk management. That's the main reason that I've been told by managers at a few banks and credit unions.
What's your idea of why some banks take so long completing an ACH transfer? And what's your favorite bank or credit union for fast ACH transfers?