@TomJNowell, it is called "push" for a reason. Nothing waits for the browser and on first byte the browser virtually gets on the resources being pushed. The big gain is that the browser do not have to wait until the HTML is received to parse it and find the resources that needs further loading.

As for output buffering, if you are zipping the html (aka deflate) you need to wait for the whole html to be generated in the first place before you can do anything. It is not done on PHP level, but on web server level, but the result is the same.

And I read enough claims that output buffering will actually make your code faster which makes sense if you have any experience with profiling on very low level. Output is always a bitch in terms of performance.