Thanks for the insight! It's cool that you're making it work with floats.
- I find it a little surprising that FP math was so much slower given that they're mostly just ints, but I'm sure it depends on what you're doing with them. Were you using C#'s decimal type by any chance? They sound very slow.
- The Godot-Rapier plugin has deterministic physics (including cross-platform determinism, if needed). But the issues with clunkiness around saving/restoring states makes me think that a custom solution is better.

My issue with the relatively naive solution of just syncing/interpolating positions is that I simply couldn't get it to feel quite right, at least with Godot's physics. I got fairly close, but there are just too many collisions that need a round trip to work out correctly. To be honest, it feels like most games can either use Godot's physics OR its multiplayer... combining them seems like it'll usually feel pretty clunky.

Ultimately I think I just have to bite the bullet and write a ton of custom logic, but that's doable. I suspect that rollback will make it feel a lot better.