Performance Tuning Challenge

I'm currently reading Java Performance by Charlie Hunt and Binu John. It's a book that covers various aspects of Java specific performance testing. I wanted to test some of the approaches in the book, so I created an app on Github that has a series of problems. Without looking at the code, can you identify the performance problems?

Then can you find the causes of the problems? Try some fixes that does not involve any large architectural change (the architecture is necessarily poor)? Demonstrate that the fixes works (e.g. some before and after metrics)?

The code is on Github.

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