Structured Page Fragments by YouTube (2017)
github.com
github.com
It also looks like it doesn't fit the current web-meta anymore :)
[1] <script src="https://s.ytimg.com/yts/jsbin/spf-vflhSOzLf/spf.js" type="text/javascript" name="spf/spf" ></script>
[1]: https://unpoly.com/
Not sure what the criteria for "fast" is, but on my 12' MacBook with Safari, it's consistently faster (and less glitchy) to just load a new Youtube page than to let this "fast" navigation do it. Using Youtube is actually painful because of this.
spfjs lets you target multiple containers with a single update, e.g. in YouTube the video area and the sidebar with related content.