Could just be that Chrome works better in general on JavaScript-intensive sites. One of the selling points of Chrome is its lightning-fast JavaScript JIT-compiler. When Chrome first came out, the first thing I tested it with was GMail, and I found that GMail was noticeably faster in Chrome. Then I tested it with a bunch of other JS-heavy sites, including my JavaScript-gaming startup that I'd just folded in part because JavaScript was too slow to run games, and found they were all faster in Chrome.
Chrome also seems to work much better with Comet-enabled apps, which GMail is. It actually has a sane threading model, while Firefox occasionally locks up when a Comet app makes the hanging GET back to the server.
I keep one gmail account open in Firefox, and another in Chrome.
There is no difference in speed for me. Chrome stutters plenty.