Kotlin is a much more pragmatic language than many such new ones. They are building it for use in their own products including performance sensitive products like IntelliJ. For example it has an inline method attribute. This may seem remarkably low level for a modern JVM based language intended for industrial use, but there's a point to it: it lets you use lambda functions and misc features that rely on them without extraneous memory allocations. Applied consistently this sort of thing can make the difference between "a lovely language that's too expensive to use" and "a lovely language that is deployable". It's not auto-inferred because otherwise you wouldn't be able to make libraries that have stable ABIs with Kotlin.
Generally they seem to be targeting zero performance degradation over Java even when using the advanced features, which is nice to see.