I want to learn how to improve my site’s speed and also for clients. On which site can I learn these techniques?
There are hundreds of sites. However, I would suggest you to learn about Google Page Insights. Understand its metrics and how to resolve them. You can search each metric on youtube to understand them and then optimise your settings. In addition, we have sites like GTMetrix, Perfmatters, Wp Rocket etc. Thesse sites have blog sections to help you understand speed optimisation.