Having to work with Optionals in Kotlin means just using their Java API. Having to work with Scala collections means having to convert them back and forth every time you interop with Java. The performance implications of the latter are much higher.
But in the vanishingly rare case where you have a performance-critical codepath that needs to zigzag back and forth between Java and Scala... it's still not a problem, because it's easy to work with the Java collections in Scala. The absolute worst case is "just using their Java API" (the same thing you dismiss about using Optionals in Kotlin). But since Scala has typeclasses we can actually do much better than that.