It's `ToArray` itself that's the problem. Since it doesn't know how big the array will be in the first place, it has to allocate a small array, start iterating, copy everything into a bigger array each time it runs out of space, and finally allocate an array of exactly the right size and copy everything into that.
You can see the `ToArray` implementation at [0], which defers the implementation to [1]. The implementation checks for ICollection to get the exact size, but the type [2] that `Select` returns doesn't implement ICollection, so `ToArray` has to fall back to the less efficient algorithm.
Oddly enough it seems that the certificate just happened to expire today in between the posting of my first and second comment. I didn't mention it only because I'm at work and couldn't be 100% certain that it's not my organizations security service messing with things.
Was trying to open your links but I am served an expired tls certificate from Microsoft!