Out of curiosity, what are you using WebSockets for?
If you download any of the native apps (iOS, Android, Linux, Win, OS X), you can play offline. This is the magic of Scala.js. The same code that my server uses gets compiled down to JS, and the client calls it directly, forgoing the socket connection. You can also see offline mode in the browser by replacing the word 'play' with 'offline'.
Yep, that was it. Just tried offline mode and it's super snappy.
For a solely single-player game, it seems to me like being able to host everything off of a static fileserver instead of a Scala server seems like a scalability win for you and a bandwidth win for users. I don't doubt your choice and your tech stack (you know way more about your project and its needs than I do!), but I'd be super-curious to hear more about the reasoning and process.
After the feedback from this thread, I'm going to move to a "hybrid" model where the model logic all happens locally, but still fires websocket events to the server. Best of both worlds.