My general intuition is that you shouldn't expect JS to beat Java or .NET, but you should expect it to be by far the fastest among similarly-dynamic languages.

My general intuition is that you shouldn't expect JS to beat Java or .NET, but you should expect it to be by far the fastest among similarly-dynamic languages. Browsers have been under fierce competition for many years, and are backed by well-funded engineering teams, so JS performance in particular has repeatedly pushed the limits on what kind of optimizations are possible in a dynamic language. Other dynamic languages haven't had the same incentives.