Seems like it wouldn't really increase the speed at which the resources would be rejected, because the spec is that the resources can only be preloaded and not

Seems like it wouldn't really increase the speed at which the resources would be rejected, because the spec is that the resources can only be preloaded and not used: they are advisory messages only. So a compliant browser can take the hints, load the files, but only "use" them after seeing the HTML and validating the hash. Then a faster browser could compute that hash in advance, as well as precompiling the JS, etc.