Measuring the loading time of your WordPress site is obviously a critical step in optimizing for speed. You have to be able to find where the bottlenecks are and where you can achieve the easiest and biggest performance “wins”. There are numerous tools, such as Pingdom, GTmetrix etc, available for measuring the performance of your site, each of them providing a different result, which is understandably confusing. Which one is “right” and which one should you use?
The answer depends on exactly what you want to measure and the level of detail that you want. They each provide different metrics which is why they provide different results, but it doesn’t mean that any of them is more “right” than the other.
However, no matter which tool(s) you use, what’s more important is understanding what information you’re actually getting, and being consistent with the tool you use. It’s not useful to compare results between tools – for example, between GTmetrix and Pingdom. It doesn’t matter if Pingdom says 2 seconds and GTmetrix says 5 seconds. You should only compare multiple results from the same tool, before and after you’ve made some changes.