On your mark... Get set... GO! You have 2-5 seconds. If your web page does not load within that time, the back button is being pushed. The search engines know if a user clicks a result to your site. They also know if the user stays on your site or comes right back to try another listing. If your web page loads slowly this "bounce" rate is a good indicator that your site was not relevant for the result it appeared for. This is a red flag to search algorithms, that you shouldn't be showing up for that keyword. A page that loads quickly has a better shot at keeping a user on the page, and therefore shows the search engines you are in fact relevant.
This really applies to the bigger issue of long term web site optimization. If you're doing a quick fix to get higher in search results, sooner or later that quick fix is probably going to be weeded out by the search engine. Meaning that you should first approach any search engine optimization project with the mind set of "Be Relevant to Your Users". If you put being relevant ahead of being heavy on keywords, in the long term you will win out. Remember, search engines are in the business of making their results as relevant as possible, and will change their systems in the future to exclude the results that aren't.