Is there a technical detail of email that I don't know about that makes it less low-latency than a text message?

Email or text would work the same for this auth scheme anyway.

tangent: SMS doesn't have the security needed for most things. We can let it be a part of auth when combined with another factor, albeit begrudgingly.

E-mail, especially when the MTA and MUA is connected to different environments, which requires relaying, could be particular slow (minutes, hours) if the e-mail hops on busy services.

SMS can be delivered days late. It isn't any better than email.

And as I discovered in my own service: If the SMS aren't being sent - or the clients aren't getting them - there are so many parts involved that's really hard to pinpoint the problem and get an ETA for the solution..