Recently Gmail started freezing for 5-10 seconds after I open emails. It becomes completely unresponsive, no scrolling, no nothing. I can switch tabs and applications. I'm using Chrome and I haven't tested in other browsers but it's definitely happening on at least two computers.
Turning off the people widget in Gmail's settings fixes this.
Credit goes to Song Zheng for figuring this out and posting about it: http://songzdev.quora.com/Gmail-slow-and-sometimes-freezing-Solution?srid=uUcK&share=1