This is often ignored but if you look at the time it takes for your webpages to load, you will know if that is what you need to work on. If it takes the page longer than 1-3 seconds, you are out of the game. And page speed is a big factor when it comes to Google rankings as mentioned before. This is often listed as a separate entity when you’re looking at the metrics because it is an important one. 

A good tool will give you metrics for various devices so that you can work on them individually. This can then be taken to the programmers and see what needs to be done in terms of optimizing the page.