Starting June 18, 2008, Google now takes landing page load time into account on keywords’ Quality Scores.
They first announced that this additional factor would be added to the Quality Score back in March. In May, Google announced that the landing page load time info had become part of the Keyword Analysis page in AdWords. You can see what Google currently ranks your keywords landing page load time as, which will help you take appropriate action to correct if needed.
The slower the page is to load, the higher odds it will lower your Quality Score, thus costing you higher minimum bids. If a page has a quick load time, you may get a higher Quality Score, reducing your minimum bids.
The down side to the system is that even if you fix your page load time, it won’t immediately fix your quality score.
“Note that your Quality Score may change/increase gradually over a number of weeks after you improve your load time.”
Tip: There may be ways around that, including changing the base URL of the keyword to a faster loading page. This should cause the keyword to be re-evaluated much sooner.
To learn more about landing page load time and landing page quality you can read this article in the AdWords Help centre.