This adds a lot of legitimacy to the fediverse

No lol. Reddit has a near 20 year headstart.

It has huge potential impacts. There are kids who get excluded from social groups for having Android

Ahhh. Well what's better overall is gonna be fairly subjective. Php in itself was made for serving web pages with dynamic content. It came first so there's a lot of resources to learn it, more established libraries, and there are more people who know how to code with it. Ernest having experience with PHP is likely a big contributor in his choice to use it

Rust is more efficient in terms of performance because it's purpose was to replace C, a language that is "close to hardware" (not really but for the purpose of this explanation just assume it). Meaning a coder can deal with things manually that PHP normally does for them. This is a double edged sword because manually doing things allows you to find efficiency in things that something like PHP would miss, but it also gives a lot more opportunities to mess something up.

So in terms of performance I would say

Perfectly Written Rust > PHP > Badly written Rust.

The subjective part comes from how well you think the Rust is coded in Lemmy. So just choose which one you like more.

What part are you having trouble understanding?

Lemmy and Kbin basically just use the same protocol for exchanging information.

Both are similar in that they interpret and present that information in a way that looks similar to Reddit which is why you can see a community on Lemmy as a magazine on kbin and vice versa. In addition Kbin also can interpret it in a way that resembles twitter.

They both started with the protocol but the way they store, serve, and present the information from the protocol is different. For example, I think Kbin stores the information that shows who upvoted what, but I don't think Lemmy does.

Setting up an instance would be more difficult too I assume