That pretty much died out due to a lack of time but I recently attended WebPerfDays and a new feature mentioned there jumped out at me. This feature is prefetch and it has some fantastic implications for web performance.
What is Prefetch?
Prefetching is the ability to request a page even though you're not on it, in the background. Sounds odd right? Why would you want to do that? Well, requesting a page means the browser can pretty much download all the content of a particular page before the user has requested to see it so, when the user does click on a link to go to that page, the content is immediately shown. There's no download time required, it's already been done.
To enable this, all you have to do is add a link tag like so.
<link rel="prefetch" href="http://clementscode.blogspot.com/somepage.html" />
And that's it. When the browser comes across that tag, it'll initiate a web request in the background to go and grab that page. It will not affect the load time of your original page.
The implications of this for web performance is obvious. Having the content of the page available before it's even requested by the user can only speed up your website but it has to be used properly. Adding prefetching to every web link on your website will cause unnecessary load on your web server so this functionality needs to be thought about before being used. A good example of this is Google. If you search for a term on Google, the first link brought back by Google will be prefetched (feel free to check the source to prove that I'm not lying!). The other links brought back are not prefetched. That's because Google know that in the vast majority of cases, the user clicks on the first link brought back and this functionality allows Google to provide you with that page as quickly as possible.
Are There Any Other Benefits?
That depends on your point of view... I primarily work on ASP.NET WebForms applications most of which are not pre-compiled... not ideal but we have our reasons. Using prefetching enables us to request pages before they're hit which, if it's the first time that page has been hit, forces it to be compiled. So we're improving performance two-fold. That initial compilation time has now been taken away from the user and we're getting the usual benefit of prefetching so users are presented with a page almost instantly after clicking.
That Sounds Awesome But What Are The Downsides?
Well, you're requesting additional pages, as long as the user actually goes to that page then that's great but, if they're not, you're placing an additional load on your server that serves no purpose.
Also, if you're gathering website statistics such as number of page hits and such then this will throw those stats off as technically, the user may not actually view that page even though it's been requested.
Finally, this obviously uses client resources, where as this may not be a problem on a nice big powerful desktop, it may be a problem on small mobile device.
And that's about it. Another great addition to the HTML5 spec. As with most things in our world, you need to think about its use rather than just blindly prefetching everything without any thought of the disadvantages of doing so.