When I notice a difference in responsiveness between Chrome and Firefox, it's usually due to the webpage being Chrome-optimized. I don't know why: maybe it was developed and tested under Chrome, maybe it uses features that work better on Chrome because they were developed and pushed by Google, or maybe the differences between the rendering engines favor Chrome.

Google's own websites are some of the major culprits. The funny thing is that similar webs don't have this issue, and I don't notice any slowdowns that push me to do side-by-side tests.