Long winded opinion, TL;DR included at bottom:
To be frank, I don’t really understand what people mean these days when they ask about performance comparisons between two Linux distributions. Unless you have scientifically accurate timing needs around performance (does your lively-hood require knowing if a process completes in 100 milli or 300 milli), I haven’t seen any evidence that there are wild inconsistencies in how one distribution implements a common software stack. Most major differences between distros tend to end up being that one has a newer kernel, or mesa version, or similar. I’m assuming that if you use Arch or Nix your software stack is going to be the fundamentally similar: Systemd, Gnome/KDE, Mesa, Wayland, etc, and neither of those platforms makes decisions that fundamentally change how those stacks perform for the average user. Both environments allow you to choose different kernels with different scheduling priorities (zen, rt, etc) if that’s your cup of tea. If you are the type of user who is capable of noticing performance deltas between compiler flags then neither of those platforms is the one you’re looking for as you likely need even greater control over the build of your software stack and should be looking at Gentoo, LFS or similar.
Arch and Nix both excel at delivering only the software you intend to be on your system along with their dependencies while also giving you a very extensive software repository (assuming you use AUR). Their largest “downside” is that they aren’t the default target for many software projects build packaging. The drawback to Nix is that it requires you to change some ideas about how software should be managed (imperative vs declarative models) and not everyone is interested building that understanding for a personal device.
Personally I believe that declarative systems and reproducibility are the future of software systems at every level of the stack and Nix is a pretty good example of how to execute on those ideas, which is why I’m using it these days. It also folds in useful ideas around multiple generations of software living in the tree which allows for complex version mixing with dependency management, rollbacks, and graceful failure of software updates. I suspect there are more elegant way to implement these ideas than how Nix has done it, but I haven’t seen them yet.
Here is one area where Nix might have an edge on Arch, Nix should handle long hiatuses with grace. Arch usually will too, but occasionally Arch has a breaking change that requires you to be aware and take action as part of an update. Even if a Nix update fails, you are always left with a functioning system (I’m not saying its impossible to mess up a nix update, but Nix is designed to not throw away the previously working state by default and to rollback if there’s a problem).
TL;DR
You wont find any real drawbacks on any of the platforms discussed, except items that, in most cases, boil down to personal preference and learning curves. Use what feels good, experiment with new ideas if that’s fun or interesting to you.