Can Bundler be as fast as uv?

If you can consistently do that, then it IS the correct thing to do.

uv made that choice and users use them, is there an objective truth of what is “correct” to do version parsing?

One challenge, as I see it, is that there are three kinds of Ruby projects that need to take different approaches to the matter, in increasing level of complexity:

(1) Developing a longer-lived, deployed or distributed project. Here you should definitely use both the Gemfile Ruby version and a .ruby-version file. You're normally only targeting one version at a time, but contributors and/or users are very unlikely to somehow accidentally end up using the wrong Ruby version without getting a very obvious notification that they are using the wrong Ruby version. That's annoying to encounter, but not difficult to solve once you know that the concept of a "version manager" exists.

(2) Hacking on your own small project or just banging out a script. You just want to run some Ruby version and get to it. You probably should default to the latest, unless there's some dependency requiring a lower version, and you might not know that until after you've gotten started. The inverse issue might also occur, e.g. you installed Ruby 3.1 a few years ago, you start hacking, and now you want to pull in a gem version that requires Ruby 3.4. You can manage this by putting the Ruby version in your Gemfile, or using a .ruby-version file, or both, but if you're relatively green and just diving in, this might not be on your radar.

(3) Developing a gem. You probably want to test/validate your gem across multiple Ruby versions, and possibly even different versions of your dependencies. You obviously don't want to lock yourself into a single Ruby version, and use of a .ruby-version file is inappropriate. There is tooling to do this, but it takes some learning to be able to utilize.

My belief is that it is worth it for install documentation for category (1) to be a little more explicit about how to get up and running. For category (2), I don't know what the right answer is, but I understand the potential pain points.

What I was most curious about is whether AlphaSite's complaint was with a specific version manager, or the fact that multiple options for version managers exist, or even the need for version managers at all?

I think the underlying issue is much bigger than merely contained on speed. It also has to do with control - both for developers as well as users.

This is also why I think the label "prematurely" is weird, because it attempts to singularize everything down to the speed label. I think the issue is much wider than what is faster. In fact, speed doesn't interest me that much; whether it takes 0.35 seconds or 0.45 seconds or even 1.5 seconds to install a gem, is just so irrelevant to me. But, for instance, high quality documentation - now that is an area where ruby failed in epic ways. Not all projects but many. And then ruby wonders why it is dying? And nobody wants to focus on these issues really. So the issue should really be viewed much larger in scope than "merely" "speed is the issue". I mean ... matz focused on speed in the last 10 years. Ok. Ruby declined. Well, perhaps it is time to focus on other things more - not that people mind a faster ruby, but if a language is dying, then you need to think really hard, come up with a plan, multiple ideas, and push these hard. Rather than, say, purge away old developers and claim nothing happened here and all is well ...