Don’t telegraph that information, I think. Better perhaps for the automatic approval to look like a fast human and the manual approval to look like a slower one. A process doesn’t have to be manual to look manual. The goal here is to reduce the cost per request.
1. Sign up with Gmail 2. Verify email 3. Account is instantly approved
1. Sign up with sharklasers 2. Verify email 3. "You're using a weird email provider. Mods will look at your account and see if it looks OK. If so, we'll approve it"