Performance Testing with K6 The problem is not that testing is the bottleneck. The problem is that you don’t know what’s in the bottle. That’s a problem that testing addresses. — Michael Bolton, author, “Rapid Software Testing.” You won’t find a lot of engineers out there who say they get excited about testing. Quality assurance (QA) is the kind of unglamorous chore that developers often approach with reluctance—like leg day at the gym. But forgive me if I wax enthusiastic about k6, the performance testing package that is the latest addition to my tool belt. K6 is a modern open-source testing tool written in Go. However, it provides JavaScript libraries to write test cases, so engineers and testers can use the k6 libraries to test their APIs and applications.