ACH is US Banking only. You can not do Direct person to person transfers, you must go through a US Bank for ACH

ACH is also SLOW and painfully insecure relaying on the banking institutions to stop gap the security and its built in slowness to allow for reversals of fraud or error transfers

ACH is not a model people should be promoting as good method to do money transfer

Thanks, I didn't know ACH was US only. As for slow, I've had things go through literally instantly, where I make the transaction, login to my bank account, and see it posted. When it's been slow, that always seems to be a factor with the sending entity (like PayPal) not my receiving bank. (Could be bank specific though)

Might be something else going on (wire transfer, maybe?); ACH payments are batch-processed a couple times a day, and so can't be instant.

Bank transfers don't have to be slow. Within the UK they are almost always instant.

https://en.wikipedia.org/wiki/Faster_Payments_Service

Does it work internationally though?

Edit: Or I should say, “Could it work internationally?” Because afaik it does not currently.

Not across countries.