As someone who worked on a smaller, but still millions-of-users live-service game, profiling of average resource usage and the loading screens was done periodically even if there were no code changes to the area of the game in question.

Given that our team was over an order of magnitude smaller than Rockstar, I would be very surprised if they did not have anyone even casually browsing a profiler every 3 months or something, though I think at their scale (LinkedIn claims 5k employees) they can probably have a team or two where everyone's entire job description be performance maintenance.

We definitely profile "dev startup" a lot, it directly impacts team speed and ability to work on the codebase. Dev startup usually includes a majority of "user startup" too, so it helps -- but it's also really easy to overlook stuff like "massive mods/mod dirs" and the like.

Speaking from experience... Once, I was so annoyed at a "startup UI"-related dialog taking way (wayyyy) too long to load. I dropped an expletive into Slack and, not much later, I was guided toward fixing my own ~2 year old mistake. We were scanning some "user-generated files" in a way that worked fine but scaled horribly -- the operation went from multiple seconds down to milliseconds. Ugh.