Performance is a huge subject, but it’s not always a “back-end” or an “admin” subject: it’s a Front-End responsibility too. The Front-End Performance Checklist is an exhaustive list of elements you should check or at least be aware of, as a Front-End developer and apply to your project (personal and professional).
As a front-end developer, you are currently implementing a layout and the customer writes you that a few things are still not working properly. You cannot reproduce the problems and want to know which browser (and which version) and which operating system they are using. No problem. Even if your customer is not very technically experienced, he can get all desired information from the website “What’s my browser” or “mybrowser.fyi” and then tell you.