I have a very basic WordPress site and, using www.webpagetest.org, I can see that my site takes around 4 seconds before it transfers a single byte. I’ve already fixed delays associated with https redirection and caching. What is it doing for these 4 seconds?

The performance test tools are great for looking at transfers between client and server, but how do I analyse what WordPress is doing?

Maybe it just takes that long to process WordPress and all the plugins but I suspect that something is wrong. Maybe a Db query is inefficient, or maybe a php script is timing out?

What is the best method/tool to investigate WordPress internal performance?