That's basically every aspect of their service. The founder Thibault Duplessis is criminally undercompensated (his choice) for running a site that is better designed, faster, and more popular than 99% of commercial websites out there.
TLDR if you want to see production-quality Scala code that this very second is serving 40k chess games -- and mostly bullet/blitz where ms latency is of course crucial -- definitely take a look.
Not as much hype for the language at the moment over Rust or Kotlin, say, but it remains my language of choice for web backends by far.