Advertisements generate revenue that helps us continue to invest in our web mail application. Features like spamBlocker and our recently launched Anonymous Email Addresses are possible in part because of that advertising revenue. There is no setting that allows you to turn the advertisements off in Web Mail. Of course you could bypass the advertisements completely by using software like Outlook Express or EarthLink Mailbox to check your email. But then you wouldn't get the extra benefits and features of using Web Mail.
Advertisements shouldn't slow down Web Mail, as they are programmed to always load last and all features are fully visible and usable without waiting for an advertisement to load. You can click any feature in the page and use it whether or not an advertisement has loaded, and you don't have to wait.
The ads have been a feature of Web Mail since we first launched it several years ago, and they have never been optional. Web Mail is one of the products that we offer free to everyone. But as a paid user you also get Internet access, email service using any software you like (without ads) and all the other things that come with the service, and you also get the convenience of using Web Mail. We understand that ads can sometimes be annoying, but we work hard to maintain a good user experience even with them.
UPDATE - On June 11, 2008 we added an Ad-Free Web Mail Option. Read about it here.