What a bizarre thing to say: not every optimization is imperceptable by humans (jpg, gzip, brotli, JS and CSS payload bundling and minification, etc. etc.) and not all sums of optimizations add up to "something significant in terms of human perception".
HTTP/3 is a good optimization, and you can't sell it based on "it improves things for humans" because it doesn't. It improves things for machines, and given that essentially all internet traffic these days is handled by large scale machine systems, that's a perfectly sufficient reason for adoption.
For a long time all my internet connections were bad (slow, unstable or both). Compressing HTML/CSS/JS, avoiding JS unless absolutely needed, being careful with image sizes and formats, etc, helped a lot... so I guess this makes me biased.
Today I have fibre at home, but mobile networks are still patchy. I'm talking sub 1Mbps and high ping/jitter sometimes. So you can see why I think an "irrelevant" optimisation that removes 300ms from a page reload, no compression vs brotli/zstd, jpg vs avif, etc, are important for me, a human.
It's important to keep in mind that many users out there don't have a fast and low latency connections, at least not all the time. What takes 300ms to complete on our fast machine and fast WiFi at the office might take 1s on someone else's device and connection. It's harder to understand this if we only use fast connections/hardware though.
That was my point: 300ms sounds like a lot until, like me too, you're on a slow connection and those 300ms on the entire 10 second page load are utterly irrelevant. You were already expecting a several second load time, that 300ms is not something that even registers: the HTTP negotiation on a modern page is _not_ what you're noticing on a slow connection. You're noticing literally everything else taking forever instead.
3% speedup is still pretty good. (especially because with some of the awfulness, it's possible to get bottle-necked by multiple of these in which case it could be 6 or 9%)
omfg: YES! YES IT IS! But you won't notice it and so the argument that it improves the experience is nonsense because you as human WON'T NOTICE THOSE 3 OR EVEN 6%
It's good because it speeds up the overall response by a measurable degree, not because it makes the experience better. That only happens in conjunction with tons of other improvements, the big ones of which are completely unrelated to the protocol itself and are instead related to how the page is programmed.
How is everyone this bad are understanding that if someone claims A cannot be justified because of B, that does not mean that A cannot be justified. It's near-trivially justified in this case. This crowd really should know better.
HTTP/3 is a good optimization, and you can't sell it based on "it improves things for humans" because it doesn't. It improves things for machines, and given that essentially all internet traffic these days is handled by large scale machine systems, that's a perfectly sufficient reason for adoption.