[malnourish]

My living partner transfers me money via ACH each month to share our expenses. It's effectively instant.

[dmoy]

ACH isn't instant, but if you use the same bank they may use something besides ACH to affect the transfer.

ACH is... FTPing around flat files, at some point in the day. Or the next day. Plus time for clearing.

FedNow is closer to instant

[otterley]

For the purpose of making rent payments, it doesn't matter.

[chews]

You've undoubtedly done this before the IRS/Treasury $600 dollar reporting requirements... try to do this with someone new.