Should this be configurable and behind a feature flag, as some Phorge installations may not want this (plus may slow down performance a little bit)?

Yes, please put it behind a feature flag. I'm not sure if we should enable it by default though.

I don't see the point in having this feature. What is it for?

The hovercard already shows the Join date of the user:

I've looked down some of the links of this request, and it looks like the definition of "new user" should be install-specific, and open to interpretation.

I suggest this path forward, for installs that are interested:

• Use Badges for the implementation; That's more-or-less what it was designed for in the first place.
• Use a scheduled task to run over the user list once in a while, and update the Noob badge based on internal logic.

This way, there's no extra performance cost when loading pages, as the "noob" flag is already pre-calculated.

I think an extension could be created as a reference for this kind of cron job to update the badge; Or an external job could use Conduit to do most of this (albeit slowly).

This might also be related to https://secure.phabricator.com/T3980 "Support "Bind to External Thing" to define project membership"

I agree, badges seems like the right way to go. They are an underutilized feature really, IMO.