KDE is nice and all, but it needs to mature a little. They can start by addressing the numerous crashes Gwenview generates on rather simple images (yes, I filed dozens of bug reports - I've never had this happen with Gpicview). Or after I close a random KDE app (especially when the "about" dialog was last displayed), it crashes ungracefully and pops an error message. That alone kills the user experience, especially for new users. I don't know about programming, otherwise I'd help them stabilize these issues myself. :(
I disabled nepomuk and akonadi in my KDE install. They were taking up a lot of resources. Without them running, everything is very snappy. Maybe that's the OP's problem?
I have kde atm running and nepomuk uses only about 25mb ram and akonadi uses about 80mb ram.
I can run KDE 4.7 on a laptop with a 1.8GHZ single core 32 bit Sempron CPU, 512MB of DDR, and a 4200RPM 70GB IDE hard drive. I tried running LXDE and XFCE on it and found a 50MB memory savings at the expense of incredible amounts of functionality. Besides the aforementioned recent files/programs, the LXDE menu had no search function, the file manager paled with the freeware replacement I'd used on Windows XP, and both LXDE and XFCE were anemic compared to KDE in terms of customization. I was unable to reproduce the power settings I had in KDE, and even configuring the touchpad was a nightmare - something as simple as configuring tap to click was very difficult, with one of the two refusing to remember the setting change and needing it to be done again with every boot. Neither had the ability to disable the touchpad while typing, either. The list of simple features that were missing went on and on, let alone missing KDE's advanced features - places/bookmarks accessible from file open/save menus, etc. I found both to be too far backwards for too little gain. If I saw no worthwhile gain on a 512MB antique laptop, I can't imagine why anyone would want to give up KDE on an average desktop.
I've been using Linux full time for just over 2 years now (as of Wednesday), which means 8+ hours a day of home work use on top of using it on my own time... and since I no longer own a TV, my KDE desktop does double duty as my TV. I've yet to experience this "sluggishness" you speak of, and it's certainly not unstable either - maybe you've only used it on a distro that ends in "buntu"? :-) I assure you I've been getting work done this whole time... data mining, programming, and assorted number crunching and data processing with RapidMiner, LibreOffice, PostgreSQL, SQLite, Free Pascal, Python, R, mathomatic, VirtualBox, GNUCash and others, along with personal use software such as SMPlayer, VLC, XBMC, Banshee, calibre, etc. Today I had a power problem that resulted in the power dropping just quickly enough to cause the system to reboot on two occasions within a half hour and I was very impressed to find that Linux, ext4 and KDE were able to recover very quickly and cleanly to the point you'd barely notice the extra reboot time and there were no anomalies with the desktop once I was back in. Firefox remembered what I was browsing and LibreOffice was able to recover an open file as well.
KDE is the last of the major desktops that still looks like a desktop and capable of competing (and beating) Windows 7's desktop on features and customizability. I don't think you should be unfairly beating up on it when it's doing an amazing job. The fact that a Windows user can take to it like a fish to water (and discover its own unique features later on) is a definite plus.