Ah, speed... Speed here is not (just) "raw speed" but also "UI responsiveness" (BeOS users will 'get' that term better). Who doesn't like responsiveness and smoothness when using a desktop environment? Everyone does, and everyone complains when they get lags between clicking a menu, or a button, or loading an app...
I have used all these DEs on new and old machines. I used MacOSX on G3s and on dual G4s. I used BeOS from a AMD K6 300 Mhz up to 1.6 Ghz machine (which is already overkill for BeOS). I've tried Windows XP on a dual Celeron 533 and on faster machines. And KDE/Gnome on the AthlonXP 1.6 Ghz and on my dual Celeron 533 (multiple distributions, Linux or FreeBSD). I also used Gnome on a $3000 machine SPARC and KDE on a SGI $18,000 Fuel machine last year.
BeOS clearly wins on UI responsiveness because of its extreme multi-threaded nature and its fantastic kernel scheduler (which plays very well with the app_server). Everything is just snappy. I run BeOS on a 1.6 GHz machine and it is simply overkill. No matter what I do, there is always CPU left for other things. I primarily run BeOS on a dual Celeron 533 and it is also extremely snappy. You click a window and while this application might have another window doing something else, nothing gets paused to wait for the other window to finish what it's doing. Everything is just readily available immediately. Make no mistake, BeOS is not a fast OS when it comes to server operations (except if you have installed the BONE networking stack), or when you do heavy compilations, and despite popular belief, its SMP scaling could be much better. But being a "multimedia OS", the engineers over at Be had special-cased a lot of things, making latency and UI a snap. BeOS "feels" fast to the user perspective, even though some under-the-hood operations are not really as fast as Linux or Windows XP. Check out this very recent YellowTAB Zeta DivX video from CeBIT, showing what BeOS can do and how fast it can do it.
Windows XP would be my second best regarding UI responsiveness. It is already very responsive, a huge (and I mean HUGE) improvement on multitasking/multithreading over the Win9x codebase, but it is not as good as in BeOS. The user can get a lot of freezing under some special cases until a window finishes what it's doing, while when I want to save an attachment with Outlook Express and I navigate to the "Desktop" entry of the filesystem it takes up to 10 seconds to read the whole root dir and refresh that window. Or when I right click on the desktop and navigate to "New", the submenu takes up to 3 seconds to open on the dual Celeron if it is not already in the cache. That's slow but they seem to be special cases which can probably be optimized easily.
MacOSX is the slowest of all in my opinion. Even on the fastest dual G4, scrolling and resizing a window (or a web page on ANY browser) is jerky and imprecise. Also, when IE or some other app is doing something, the menu bar on top doesn't respond and all I see is the spinning beach ball. There is no real responsiveness there. In fact, speed is my number one problem with the otherwise excellent OSX. As someone else said once "after you have used BeOS, anything else will never be the same and it would just seem slow" and that has proved true.
Gnome is a bit faster than KDE. While GTK+ 2.x is several times slower than GTK+ 1.x, it still manages to run adequately well (however Metacity is so slow when resizing an window it will redraw its button bitmaps in a really ugly fashion). KDE is plagued mostly of the fact that all its applications are slow to load. In fact, even the smallest Gnome application (e.g. calculator) is slower to load than the big and fat Blender because of the number of shared libraries is linked against! But KDE's performance on loading its apps is worse. Overall, these Unix DEs are based a lot on how the kernel is configured (same goes for the other DEs too though) and I found that the default FreeBSD 4.x ran KDE much faster than a stock Linux 2.4.12 last year. I remember Gnome 2 running slowly on that brand new Sun SPARC workstation though. So, there is definitely some responsiveness to be gained depending on which platform you are running or how well optimized your X or kernel is, but on the stock Mandrake/SuSE/Red Hat distros these DEs ran from well to slow-ish. They could be better, but right now they are definitely usable.
Rating: Windows XP 9, MacOSX 6, KDE 7.5, BeOS 10, Gnome 8.
Stability and Bugs
I found Windows XP and MacOSX to be the most stable environments in this long term test usage. Sure, I have seen both Finder and Explorer crashes (which are the "desktop shells" for these DEs, as Nautilus is for Gnome and Tracker is for BeOS), but overall stability is good. I think XP has fewer bugs than MacOSX though (has anyone seen the OSX Font panel not showing the preview of the font selected? You need to manually drag that panel down..). I had a few bugs with XP's taskbar in its first versions (it was freezing after a cold reboot for about 5-6 seconds) but they have all been ironed out now after the SP1. In fact, the most notable bugs I have to report about XP's default apps is how sucky this Notepad application that I write this article right now is.
Gnome 2.2.0 is somewhere in the middle regarding stability (especially Nautilus which crashes easily). It has a number of bugs (especially Nautilus) but if the guys are able to iron out Nautilus more, Gnome is pretty solid overall. And the good thing is that if something is wrong, you just kill X and you reload. Update: Gnome 2.2.1 seem to have been a stability and bug fixing version mainly for Nautilus.
BeOS and KDE are the most unstable of the lot. BeOS Dano is a beta, but even on BeOS 5.x I could crash the app_server of Tracker easily (both part of the DE part of the OS). BeOS is pretty stable overall, but if you "overwork" it or do weird stuff on it can crash easily too (that doesn't mean that the whole system will go down. If Tracker goes down is easy to reload it, but if the app_server goes down, you'll need to reboot).
As for KDE, well, Konqueror is just not stable. Applications written or coming with KDE also tend to crash fairly easily (on a number of machines and different distros/OSes I ran it). However, the biggest problem with KDE is not stability, it is the bugs. Konqueror has more bugs than the whole BeOS does. I just can't stand it being so inconsistent, bloated, buggy and crashy. Poof. It's gone.
Rating: Windows XP 9.5, MacOSX 9, KDE 7, BeOS 7.5, Gnome 8.