Painfully slow Feed Me import of User accounts
Turns out the answer in my case is to use a JSON data source.

Importing users as .json instead of .csv appears to be about 10 times faster: 


With .csv importing 480 users took 2.5 hours
With .json importing 455 users took 15 minutes


My JSON looked like this: 

{"user": [
    {
        "username":"jane.doe@example.com",
        "email":"jane.doe@example.com",
        "firstName":"Jane",
        "lastName":"Doe",
        "password":"User pass phrase",
        "groupName":"User group name"
    },
    {
        "username":"joe.bloggs@example.com",
        "email":"joe.bloggs@example.com",
        "firstName":"Joe",
        "lastName":"Bloggs",
        "password":"User pass phrase",
        "groupName":"User group name"
    }
]}