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Also the kotlin example is using coroutines

Coroutines in Kotlin can be convenient, but they are overall about 20% slower than non-suspending code. I didn't spend too much time investigating the cause, but I believe it's because of the extra object that is passed along in all method calls.

In my case, I have a programming language interpreter implemented in Kotlin, and as an experiment I made the entire interpreter using suspending calls so that I could call asynchronous functions, and my performance tests dropped by about 20%.

As I understand it, Kotlin coroutines generate what is known as irreducible control flow. This means that loops have more than one entry point, or equivalently, there are loops that have a goto jumping into the body from the outside. (Java the language and I believe also Kotlin the language don't have goto, but the Java bytecode does.)

Irreducible loops make many optimizations much more complex. Bytecode generated by javac never contains irreducible loops, and since bytecode generated by javac is the number 1 use case targeted by JVM JIT compilers, they probably just don't bother trying to be that smart about irreducibility.

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