The short story is that native promises are faster now except for the "promisification" part.
The benchmark was designed for realistic use in a node environment, where most of the libraries come callback based. Because of that a very fast "promisify" is really important. Native promises don't provide one so the naive implementation using standards-compatible API is quite slow.
Bluebird's promisify is a lot faster since it relies on non-standard (as in non-ES6-standard) internals instead of using the promise constructor as an ES6-based promisifier would need to do.
edit: on second thought, I haven't looked at the included `util.promisify` - it could be taking advantage of non-public internal V8 promise APIs.