Javascript and PHP scripts loading time effect for SEO

We all know page loading time matters,

Let's say I have some random username/number generating page to the user.

Do I need to write it on PHP or javascript?

What I'm expecting, as a result, is: when frontend page combines with the above script, it must be loaded in quickly as possible.

So in this case, what to choose from your experience?

Also please share some info/keywords/links that i can refer and learn more about optimization.
i found this: google's recommendations: https://developers.google.com/speed/docs/best-practices/rules_intro

Loading times aside, for SEO purposes you usually want to render as much of the content on the server as possible. Bots will have trouble crawling content that requires a javascript client.

Beside seo ,you will have trouble with Adsense if you use ajax