Without a doubt Bluebird, When.js, Q, then/promise and others are faster. The implementation in httpinvoke is optimized for code size.
The other reason for current solution is that the specification for creating and resolving promises [1] is not yet published.
Having a Promise object in global scope, as exported by Q, Bluebird and then/promise, is not enough. There is no uniform way to hack into these implementations and use their promises resolver.
But when resolvers-spec is available, httpinvoke will definitely check for already loaded promises library and reuse it.
[1] https://github.com/promises-aplus/resolvers-spec
