I’m not sure why nix-env is so slow exactly but it’s the wrong tool to use anyways as it just throws away everything NixOS has going for itself in favour of pretending to be a normal package manager. You really just want to use the configuration file.
The “normal” way to install packages in NixOS would be using environment.systemPackages. The various programs.<name> options are intended for packages requiring additional setup, like shells or desktop environments (e.g. iirc for sway it creates a systemd target and adds the .desktop files for login managers to see it). Weston has a package but not an option, so you’d have to figure that additional stuff out yourself (but running Weston from a tty should just work).
There are additional ways to install packages for single users or using home-manager but you don’t need those.
This does kinda demonstrate why (I personally think) NixOS is so hard to learn: There’s a million different ways to do anything and each has it’s own weird gotchas. And critically most of them, even when they are honestly just legacy cruft, are not actually deprecated and may even have users advocating their use, or even if they don’t nobody bothered to remove that part from the wiki (if it was ever there to begin with).
You can also see this in the flake/channel split: One person in this very thread is telling you to use flakes, while another is telling you to stick with the default (channels).
And in some (fortunately relatively rare) cases even things that everyone agrees are bad ideas still get promoted in official documentation or other prominent places, like using nix-env -i under any circumstances, ever.
And it is definitely a learning problem you are having. You are facing the same problems as a new Linux user who just installed Manjaro with KDE 6 on Wayland and is wondering why apt-get and xrandr are not working even though those are accepted answers on Stackoverflow posts from 2012. Of course as experienced Linux users we know why, but a new one has to learn a lot of stuff before “getting it” and will probably stumble onto poor advice more than once in their journey. (And learning Nix is arguably worse than learning Linux for the first time, but that depends a bit on your exact experience and background.)
If you stick to learning NixOS there will be a point when these things seem trivial, but it will be a lot of effort to get there. Is that effort worth it? Well, if the term “declarative package management” doesn’t mean anything to you, maybe not. You do sacrifice a lot of things “just” to declare your entire system state in one configuration file (or more likely, directory). But I do think the things Nix does are really cool, if you can get over the, uh, everything.
Also coming from Arch will induce a bit of a culture shock regarding documentation as the NixOS wiki is just… not very good. It’s neither complete nor reliably accurate for the current release. And some wiki pages are actually just snippets with no explanation for either what they do or why they do it.