file sizes and speed

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Since the early days of the web, users have been complaining about the time it takes to download web pages. The phrase "World Wide Wait" was coined to describe the frustration users felt at having to wait for ages for a web page to appear. Internet access speeds have gradually increased. So is file size still an issue?

how fast would users like to download files?

Users would probably like their web browsing experience to become as fast and seamless as off-line work on their PCs. The web usability guru Jakob Nielsen suggests that there are 3 important levels of response times when considering the speed of response of a computer system to a user’s action:

0.1 seconds or less gives users the impression that their system is responding instantaneously
1 second or less is just about fast enough for a user’s flow of thought to remain uninterrupted, although they will notice the delay
10 seconds is about the limit before a user’s attention will switch to something else

Nielsen warns that you should allow about 0.5 seconds for “internet latency” – the unavoidable delay inherent in the infrastructure of the internet. This makes the first target, a download time of less than 0.1 seconds, unattainable for delivery of web pages over the internet. For web sites, a reasonable target would be for users to be able to download your pages in less than 1 second. This delay will be noticeable but will allow users not to lose their train of thought.

so how small should web pages be?

Taking the target of a one second download time:

users with 512 kbps broadband could download files of about 30 kb in size
users with 56.6 kbps modems could download only about 3 kb (typical download speed for a 56.6 kbps modem is 52 kbps)

It is worth noting that many web pages are around 100 kb in size and so would take much longer than the 1 second target for both broadband and modem users.

A good target is to keep total file size (HTML page plus logos/images plus stylesheets) to less than 30 kb, which will take:

1 second to download using a 512 kbps broadband connection
5 seconds to download using a 56.6 kbps modem

what do you mean by file size?

When considering download speeds, it is important to remember that there may be several separate files that must be downloaded before a user can view a web page. These can include logos, other graphic files, style sheets and multimedia files. In this note, we are always referring to the combined size of all associated files that a user has to download the first time they visit a site.

won’t broadband mean that file size is irrelevant?

The wider availability of broadband connections of 512 kbps or faster does not mean that file size is irrelevant. If your web site files are kept small, those users who do have broadband will experience very fast downloads. But don't forget that the ideal response time is less than 0.1 seconds! The number of UK homes with broadband exceeded the number with dial-up connections for the first time in 2005. But it will be a long time before all your users have broadband connections. In the meantime, file sizes should be minimised to provide an acceptable response time for users with modem connections.

doesn’t cache speed things up?

Web browser applications use cache to store web pages on the user’s PC so that, on a return visit to the same web site, the pages can be viewed from the cache rather than downloaded again over the internet. Many users will have cache configured for use only for subsequent visits to the same site on the same day or in the same browser session. So, on any given day, a user’s first visit to a site will download pages direct from the site and subsequent visits in the same day or session will take the web pages from cache. So cache does help to speed up the browsing experience. But for many users, pages have to be downloaded in full over the internet at least once a day – more if you have closed your browser down at all during the day. If your page downloads very slowly, most users will have to endure this delay at least for their first visit any day to each of your pages. So designing your web site with small file sizes will give you a clear advantage.

the need for speed

Users clearly benefit from faster downloads. A good target is to keep file sizes below 30 kb - but this still means considerable delays for modem users, who would be better served by file sizes as small as 3 kb. Compare your site with our 18 kb home page or Google's 13 kb home page - and see our article about a hypothetical search engine race. The smaller the file size – the better!

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