As soon as you publish your site you should make sure that your site is as fast as possible. Slow sites may not necessarily make your SEO rankings drop, but is weighed as a factor, as you may read in this blogpost by the Google Webmaster folks. The same blogpost also refers to internal studies that showed that visitors spend less time on slow sites. Also, if a visitor tries to access your site and it takes more then a few seconds to load the site, he might leave the site before it loads. This is registered as a bounce by Google, and will raise the bounce rate. A general rule of thumb is to keep the time needed to load your site under 2 seconds.
Research has also shown that conversion rates might drop if you have a slow site, slow being anything over 2 seconds. Moz.com hosts one of the major SEO blogs, and they published some interesting stuff on this topic:
Moz.com: Website speed and search ranking
Moz.com: Website speed and conversion
Make Joomla fast!
Out of the box Joomla may not be exteremely fast, but it takes just a bit of configuration to make it dramatically faster. You may easily reduce time to load your website with 50%. Just check the articles in the Speed-checklist, or if you prefer a small 5-minute video, jump to our blog article about speeding up Joomla in 5 minutes.
If done well, you might achieve the same score as we did: