Are there any performance issues that are client of your own API?

Take Twitter, for example, twitter.com, as your API client. Could this be one of the reasons that Twitter is pretty “slow”?

Link: http://engineering.twitter.com/2010/09/tech-behind-new-twittercom.html

Would you recommend using your own API for your main site / application?

If using the native API is ok, what are the ways to avoid performance issues?