Kawa fakes it. The JVM doesn't support it so if you use a normal function call you don't get tail recursion. You could avoid calling functions and instead do y

Kawa fakes it. The JVM doesn't support it so if you use a normal function call you don't get tail recursion. You could avoid calling functions and instead do your own stuff but that doesn't change the fact that the JVM doesn't support it. That is what they mean by performance, you lose performance because you can't do naked function calls in those cases. Since except mutual recursion (which is difficult to detect tail recursion for correctly) the benefit of tail recursion is a performance boost it is ignored when the cost of implementing it kills your performance.