Running KDE on a dual core atom netbook with 2gb of ram (no ssd), and not experiencing any of the sluggishness you're talking about. In addition, I've added the kxstudio repositories, run a full JACK setup (considerable CPU overhead), and a digital audio workstation (ardour3) along with fl studio in a wine session. I plug in a second monitor, and I still have none of the problems you talk about. Granted, I'm only using stock widgets, other than Appmenu QML, but performance is snappy. On top of all that, my dolphin/rekonq/amarok/clementine/usage barely makes a blip in memory usage, as all the shared libs are already loaded. But even Firefox or chrome won't make the system unresponsive.
I really enjoy KDE. I just don't like fighting it. Likely a combination of using stock Kubuntu and having tons of files in my home-folder caused the problems. I'm now using a subset from the PPA without KDE PIM and things run smooth.
I _really_ like KDE and enjoy using it most of the time. But I think it's not unreasonable to assume that it could be much better. From my subjective point of view is Windows 8.1 on the Pentium M 1.6Ghz notebook with 2GB faster when using the explorer than dolphin on a Core2Duo 6550 with 4GB RAM (with a quite fast Ultrastar A7K1000 HDD). Opening and saving files and things like that. It's still reasonable fast. I really can't complain. I just don't like the defaults and as I said I think KDE could do a lot better than that. When using Eclipse and doing Java development every megabyte RAM is worthwhile :)
I did the switch from Gnome 2 to KDE 4 when I found that KDE 4 can run smooth with VESA driver on X11 and Gnome was running like a slug. Also, KDE have tendency to run with less problem when your X11 driver is buggy (AMD drivers...)
So I'm curious as to what you're doing wrong.