If you earn a living developing websites, one of your considerations should be website performance. An invaluable tool for investigating performance issues and finding bottlenecks is WebPageTest. If you have never heard of it before, be sure to check it out and give it a try.
It may seem quite complex and confusing at first, since they condense a lot of information into a few charts. However, it is well worth the effort to learn and understand it. If you are wondering what all those charts, lines and colors mean, be sure to read this awesome article “Reading a WebPageTest Waterfall Chart” from Matt Hobbs. He explains every little piece of WebPageTest and helps you kickstart your web performance debugging capabilities with WebPageTest.