In addition to this, it's been shown that 40% of visitors will abandon a website that takes longer than 3 seconds to load.
If that still hasn't convinced you, Google is increasingly using page load speed as a measure in search rankings.
The thinking here is that Google wants you to find the page you are looking for as quick as possible. If your site is slow to load, this impacts the Google user experience. For this reason, fast sites are rewarded by Google in terms of rankings, meaning more click throughs and more potential customers land on your site.
Stop losing sales! What to do about it...
Ensure your server has enough capacity (RAM and processor) and consider upgrading if not. Use a good quality provider like RackSpace.
Check that HTML validates well as this can slow down page render time.
Make extensive use of caching, especially for content that is the same for all visitors. You could consider Memcache for this.
Ensure images are saved at the size you want to display them. You can do this manually but ideally your content management system will do it for you.
Use content delivery networks where possible to delivery common libraries like jQuery.
Switch on compression on your web server and enable Keep-Alive for resources wherever possible.
Try installing Google PageSpeed for your web server.
Reduce the size of your HTML pages, clean up your site and remove unwanted elements.
A good starting point is Google PageSpeed Insights , which gives you scores on a few aspects of loading speed optimisation. The Pingdom speed checker is also a good tool and shows you how your site compares with other sites.