Kathyl, the timer automatically starts 10 seconds after you solve the cat captcha irrespective of whether the site has finished loading or not.
If the page loads completely before those 10 seconds, then the timer starts immediately.
I think this is a fair system as you need to see this from the advertiser's perspective also. You need to give atleast some time for the ad to load else most of the ads we will be clicking would be blank pages.