After checking the source code of your “idiomatic” implementations, I can guarantee that there are many ways to write idiomatic Kotlin code and some ways will provide better performance than what you wrote without sacrificing readability.

Also I would like to emphasize that the source Java code is not idiomatic Java in the first place, it’s Java code tuned to give the best performance possible. So if you spend time to fine-tune the Kotlin implementations as well, the performance gap will be smaller.