These things are somewhat self-reinforcing. Many sites feel slower with Firefox because they've been developed on Chrome and avoid the slower sides of Chrome, yet won't take advantage of the paths in Firefox that are faster than Chrome.

This has been especially evident on some Google properties like Google Docs, or the incident where Inbox couldn't support Firefox because it implemented a function correctly where Chrome didn't.

That said, I hardly notice any performance difference on most sites, and IMHO Firefox behaves much better with a large amount of tabs. Security is the main issue, but Firefox sandboxing is starting to roll out.

The rendering performance difference is noticable on almost every site and it doesn't matter at all how they've been developed. Even sites developed in FF with no regard for Chrome features render more smoothly in Chrome.

Then there's the developer tools that slow everything down even more in FF, to almost a halt/crash on some sites, while on the same sites they have no impact on performance in Chrome at all.

That is with an i7 and a mid/low range GPU.

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.