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LINQ is parsing a tree of System.Linq.Expression here and the cases of First(pred) etc. are just not optimized because of the added complexity with little benefit. It only recently became a problem when Visual Studio got a new built-in analyzer that tells people to "optimize" this.

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If you're using LINQ on a list, it will not use Expressions, but a plain Func. So nothing will get parsed either way.

The difference, as @sbelskie already mentioned, is that Where has an optimization for List, while First only uses the naive enumerator.

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