The performance is very acceptable for what I saw (basic usage of all of the provided apps).

I am no expert, but the FirefoxOS stack is really lean and simple compared to the Android stack. Some would say that Android is an huge undocumented bloat with all the layers added during its lifetime (just check the AOSP build system to understand this opinion).

As I see it, FirefoxOS, by being ten years younger and less used, is doing million things less than Android (from the system point of view), and that's how things can get reasonably fast and responsive (that, and the blazing fast SpiderMonkey vm).