@prog We'll always place an emphasis on getting NodeBB loading quickly, as user experience is one of the features we'd like to focus on.
That said, the Google pagespeed test is at best still machine driven and does not accurately represent real-world usage.
I don't mean to disregard the test results -- not at all! As @psychobunny mentioned, we used to score higher.
Re: serving smaller images - this is unfeasible in core as we merely store post content, and do not rehost images. If someone links to an imgur image, we wouldn't intercept the image and host multiple versions of it locally (one for each dpi/device), because that is squarely in plugin territory.