How did you find the demo page, and why did you expect it to be more useful than the front page at explaining what LLVM is?
The llvm.org front page already has "The LLVM Compiler Infrastructure Project" as its title and the first sentence of the body text is "The LLVM Project is a collection of modular and reusable compiler and toolchain technologies."
llvm is the "back-end" of a compiler turned into a reusable library, where the front-end of a compiler is what parses and understands a specific language (clang or rust being examples of primarly front-end things that leverage llvm).
if you make use of llvm, you "simply" have to parse your chosen language and hand off some intermediate form bits to llvm to create compiled binaries.
By using "performant" in the title (which is known to set off the language nazis), the author is practically begging for the discussion to devolve into this tangent.
It's imprecise: What is it even supposed to mean? Because it's a made-up word, who knows for sure? Does it mean "better performing?" If so, why not just say that? It's not that many more keystrokes. Better performing in what way? CPU? Resource utilization? Say so. You've put a lot of thought and effort into writing something, why blow it by using an imprecise pseudo-word? The author is undermining his own credibility, telling us "I don't care enough about the topic to even pick an actual word, let alone summarize, in more detail, what I mean to discuss."
My understanding is that performant is totally legal French, meaning efficient or effective, with usage dating back at least 4 decades. If you're not into stealing random words from other languages, I have to question why you're into English in the first place.
Demo page is not working. Is there any other page that makes me understand what really is it and where it is helpful.