From what I’ve seen so far from people I know who run Valve Indexes, Linux SteamVR performance is pretty poor compared with Monado+OpenComposite. Hopefully this situation changes with the release of the Frame, in which case I (and likely others) will be revamping the SteamVR package and NixOS modules as Monado may not fully support it for some time.
Tl;dr: Run Monado w/ OpenComposite for the Index, it runs way better.
Tl;dr: Run Monado w/ OpenComposite for the Index, it runs way better.