In this article I will expose some of the tool we use to cover different aspects of performance of a particular website.
Gtmetrix is pretty good to spot things like slow JS loading/slowdowns and overall browser/client side load time.
WEB PAGE TEST
Another similar tool is Web Page Speed Test. You need to make sure to select the proper value under the Test Location dropdown.
Pingdom is very similar to Webpagetest and it’s a great alternative to get a different report.
GOOGLE PAGE INSIGHTS
Google offers this tool which also checks the responsiveness of your whole site and telling you exactly what to do to improve it.
If you need to scan a Magento site, Magereport can give you an overall of the whole site and telling you if you site run risks of hacking, if it has a malware on it or even if you are missing some of the Magento 1 security patches.
Sucuri offers a great tool to check if your site has a malware on it or if it has potential risks to be hacked. This is a great tool to improve the security of your website.
Here is a very good article written by Tung Tran which gives 22 amazing tips of how to improve your website performance. Take a look at it!