I find that Gmail is incredibly slow in rendering with my FireFox browser. Not sure if it is FF in general or if it's extenstion related, but I found a workaround that speeds up Gmail considerably.
I use a "User Agent Switcher Extension" for other areas of my job including SEO and debugging internal applications. This extension allows you to change the information that you send to the host website and specify whether you're using Internet Explorer, Firefox, Safari, or anything else that you want.
I found that if I use the extension to change my user agent to something like "Googlebot 2.1", Gmail reads my request as an "unsupported browser" and switches to an alternative mode which renders many, many times quicker.
Another benefit to claiming that you're Googlebot 2.1 is that you can sometimes get into password protected areas on news websites. 🙂