Out-of-the-box, ExpressionEngine works very well, but on larger sites that attract more traffic, there are several options to fine-tune that can help you keep ExpressionEngine humming.
One downside of any website that allows members of the public to submit content is spam. Spam can take many forms, including comment spam, forum spam, and member registration spam (where spammers register as new members on your site and then place links in their profile pages), Luckily, ExpressionEngine has several tools you can use to combat spam, many of which are outlined in the ExpressionEngine documentation at http://www.expressionengine.com/user_guide/general/spam_protection.html. Suggestions include:
Use the blacklist/whitelist module to keep known spammers from your website and prevent content (in comments or member profile fields) that matches anything in the blacklist from being submitted. There is a one-click option to download the latest blacklist from ExpressionEngine...