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,

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. [0] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,783a052330e7d48d https://referencesource.microsoft.com/#System.Core/System/Li... [1] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,ed118118b642d9d4 https://referencesource.microsoft.com/#System.Core/System/Li... [2] https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,cffd4efbf86815cb https://referencesource.microsoft.com/#System.Core/System/Li...