In order ensure that Scala-IO will be a performant library there are an ever growing suite of tests that are designed to measure the performance of a particular IO operation performed by Scala-IO.
In addition to the simple measurement is the "ideal" implementation which is a java based implementation that is the best possible implementation. Scala-IO isn't really intended to reach that performance goal but it should constantly approach that goal.
Also, to keep us honest, there is a history of the performance. Hopefully that will track steadily improving performance characteristics