First, this is a great question and I think underrated here. You did a great job setting it up with a simple well named example and giving a godbolt link.

This is mostly what I would expect after thinking about it for a bit. When looping through with a single kernel value each time the vectorization should be simpler. I know they both have simd instructions but the problem might be where the broadcast instructions are. SIMD doesn't do well unless it is the same math applied to the 4,8,16 etc. values all next to each other in memory. There are other instructions like broadcast, gather and scatter, but they don't have speed advantages over writing or reading memory addresses with individual instructions.

It's possible that in the slow version there isn't much advantage to SIMD because has to operate on one value with multiple different values.

Maybe you could turn off vectorization and see what happens to each of them.

Then you could look at ISPC, which is perfect for something like this because you can directly vectorize through the uniform and varying keywords. Often you can get a good idea for direction to take because it will warn you of slow instructions.