It's crazy slow. Most ACH moves through a modern stack and then into COBOL that runs nightly batch jobs that then update the stuff in the modern stack and in on its way. Nevertheless, it takes the nightly batch job.
Honestly though, they processed 22 billion payments totaling 38.7 trillion dollars in 2013.[0] When you're dealing with numbers like that, there's an argument to made in favor of battle-hardened COBOL batch processing over some real-time rewrite.
