I don't know what relevant performance issues you're referring to, but if you want to learn more about concurrency and performance, I suggest you start with some of the basics of the theory behind concurrent servers: https://youtu.be/07V08SB1l8c

As I said, I've put examples and detailed explanations in a significant amount of material that's available online that will help you understand how and why user mode threads work and why we decided to add them to Java. While I can't teach concurrency and the design of the Java platform from the ground up (especially detailed mechanisms such as stack walking, JVM TI and JFR) on an individual basis on social media, I'd be happy to answer any specific questions you may have once you've familiarised yourself with the subject.