It also looks like it doesn't fit the current web-meta anymore :)
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.