ome6a1717's method won't work: both the runtime and network latency are non-deterministic, so only sending inputs will cause the games to diverge until both sides are seeing something completely different. Multiplayer programming is exceptionally difficult, you should really use the built-in syncing features, which have carefully tuned redundancy and compensation features that can adaptively handle things like changing average latency and latency variance, high packet loss and packets arriving in the wrong order, while keeping synced object movement smooth and very close to the position on the host. If you think you can just send a few messages and get a similar system, you are severely underestimating the technology behind the Multiplayer plugin.
If you run two tabs on the same machine, you should easily see <5ms latency between them. A poor quality long distance link from e.g. UK to Australia could be around ~500ms. So if you're seeing 600ms+ on the same machine, that indicates some kind of fault. The latency is 100% to do with the network connection, WebRTC won't add more than a few milliseconds. I did extensive testing of the feature at the time, and even long-distance games worked well with the multiplayer engine's features for compensating poor quality connections.