I mean, it's the reason the benchmark is slower, because the benchmark is comparing doing 2 things with only doing one of those things -- this is virtually taut

I mean, it's the reason the benchmark is slower, because the benchmark is comparing doing 2 things with only doing one of those things -- this is virtually tautological. So I agree that it is why it's slower, but I also agree that it's a mistake. This is the "hard part" about synthetic benchmarks. This one is measuring something people rarely do: creating an anonymous function and calling it in a tight loop. It's extremely unlikely you're ever going to run into a situation where creating this lambda is in a hot path, and if you do the thing you did wrong is very unlikely to be whether you used a standard library function or one you defined yourself. It's that you have a loop where you did almost nothing but create a lambda every iteration instead of, say, passing it to a map/fold function or whatever.