I would argue that writing great asm that is more efficient than your idiomatic C code is going to be a very very challenging task on modern non-embedded CPUs. The compiler definitely knows more than I do about how the internal architecture works and which instructions to reorder to prevent stalls etc
SIMD intrinsics are not idiomatic C, they are idiomatic assembly.
Here's an example when they improved by a factor of 3.6: https://stackoverflow.com/a/63759887/126995 That's not an unusual outcome.
Winforms got a lot of hate for some reason I don't understand, but designing with it in Visual Studio is the easiest, best, most sane layout/designing experience I've ever had. It makes me wish the web was more like that.
I've seen several examples of the compiler being brilliant until suddenly a simple change makes it not brilliant at all anymore. Compilers are just tools.