Created by: ssorallen
jquery.com uses Google Analytics and so sets a cookie when you visit the site. When you request jquery.js from its CDN, code.jquery.com, you take a hit from sending your previous GA cookie in the request.
Google's CDN, googleapis.com, never set cookies and so doesn't add overhead. It also is faster in my unscientific testing of a billion Command + Shift + R's.
Twitter's widget library has an async snippet to load the JS rather than a synchronous script tag. widgets.js was before other, important JS and slowed down document ready occasionally even on a decent connection.