I swap between Firefox and Chromium (yes, -ium, not -e) when developing web-related stuff - one day FF, the other day Chromium, recent versions of both browsers. Development is done on... a 2004 vintage Thinkpad T42p. With a whopping 2GB of memory. If it works on that, it should work on anything. I swap between these two to avoid developing towards one specific rendering engine and to get a feel for how things will work in 'the real world'.
While Chromium does beat Firefox in speed in several individual tasks the overall experience does not differ all that much between the two. Chromium is noticeably more memory-intensive than Firefox, this can go so far as to have the system slow down to a crawl with only two or three tabs open. With Firefox this is much less of an issue, it handles dozens of tabs without a hiccup.
Development tools are more or less on-par between Chromium and Firefox+Firebug (or Aurora). I do not notice the slowdowns you mention when using the developer tools - at least not when comparing between FF and Chromium - even though my system is much slower than yours (1.8GHz Pentium M, 2GB, ATI FireGL Mobility T2 + 128MB). Of course things just are slower on a system like mine so maybe I'm just used to waiting those extra few milliseconds here and there?
In my opinion Firefox has a much better user interface than Chromium, partly due to the fact that Firefox uses GTK (and as such looks (or can be made to look) like most other applications where Chromium comes with its own toolkit.
Once I'm done I test whatever I made on Safari on iOS (the 'new IE6'...) and prepare to jump through some hoops to work around the problems which invariably crop up.