I run Kubuntu, and I have only really run into one bug with knetworkmanager sometimes asking for my wireless password even though it's saved in my wallet. The rest were bugs in or caused by shared components (X/intel driver, MSI Wind brightness bug, etc). I admit I have some pet peeves here and there, like there not being an auto-reconnect plugin in Kopete, no touchpad configuration tool, etc. But no other bugs, no instability.
Apps starting slower in KDE (like Firefox or whatever the example was) is indicative of lack of available resources, such as of RAM or caused by some process hogging CPU time. Additionally, in the case of Firefox, perhaps it needed to load shared GTK libraries that no other apps had already loaded, whereas GNOME would've done so early on at boot. The same would go for Qt apps under GNOME.
Being as broad as to say that "stuff is slower in KDE" makes little sense with apps started in the same X.Org with the same drivers on the same hardware, unless there is competition for resources. The environment is just another set of apps and daemons.
I think that what Kubuntu needs in terms of "distro packaging" is a good browser, as Konqueror isn't it. rekonq is a bit early in development and needs, really needs, Flashblock functionality to be a valid contender. I don't know what happened to its early ideas of multiprocess browsing, which sadly might just be too late now design-wise. Arora aims to be portable and environment-independent, which makes it integrate poorly by design. Chromium is beta (although works quite well), but integration is nonexistent and appearance is "unique" by design. Its speed and screen budget is unequalled though, for which my netbook is ever thankful. It, too, needs (really needs) Flashblock functionality, though there are extensions that sort of sometimes properly gets the job done. At least there's a built-in task manager (shift-esc) through which you can close the Flash process, in case it goes rampant.
So it needs a more mature rekonq, a Karora or a Kromium (gods yes please).
continuation: But yes, Kubuntu is in a position where considerably less attention and resources are directed its way compared to its trueblood Ubuntu half-brother. Some features that are very core to Ubuntu can't easily be applied to KDE (Apport vs Dr. Konqi). Some features that are very core to Ubuntu won't be applied to KDE for other reasons (PulseAudio). Some design decisions are made without taking non-GNOME installations into consideration (O asoundconf, Where Art Thou).