It's a shame, actually, that .NET performance improvements of up to x1000 could still be found after two decades and hundreds of millions spent on development.
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For example, the ReadOnlySpan type is a recent addition to the runtime and it will allow faster iterations when used. They simply enabled Linq to use it now.
Well, better late than never.
I think the main reason why C# didn't have this (and other low-level features like plain function pointers) for so long is because the original vision for .NET was that you'd mix and match different languages in your project as needed. So if you needed that kind of stuff, you'd reach out for managed C++ (and later, C++/CLI), while C# was kept deliberately more high-level.
And so once that approach was abandoned, C# needed to catch up to be able to play the "one language that can do it all" role.
ReadOnlySpan is so powerful because it can be used generically and has implicit conversions that allow you to improve the speed of existing methods without breaking backwards compatibility.
It's well designed and that takes thought and time.
The Nx elements figure is a speedup you would expect if you introduce a shortcut to any algorithm that can bypass doing full iteration, and >100x speed-up is what you would expect from going from interface dispatch per element (even if devirtualized behind a guard) to a bespoke vectorized implementation.
Even if you happen to have irrational dislike of .NET, this is still useful learning material and perfectly applies to other languages that expose similar functionality (provided they have sufficiently good type system and generics).