My first steps into the performance and troubleshooting world, already many years ago, were started by documents written by Sander van Vugt. Many of this information and more survived in a video course: Red Hat Performance Troubleshooting and Optimization (https://www.sandervanvugt.com/).
Another big source of information is the website of Brendan D Gregg (http://www.brendangregg.com), where he shares an unbelievably big list of his documentation, slides, videos and so on. On top of that, there are some some nice utilities! He was the one who taught me in 2015 that it is important how to identify the problem:
- What makes you think that there is a problem?
- Was there a time that there wasn't a problem?
- Has something changed recently?
- Try to find technical descriptions: such as latency, run-time error and so on.
- Is it only the application, or are other resources...