If you're experiencing lag in KDE, disable the composition.
If it still does not solve your problem, use a different widget other than Oxygen.
I also notice that some systems, mainly nvidia binary drivers enabled ones, tend to leak a bit of memory. In contrast, my Intel GPU netbook idles nicely at around 170MB after lots of programs have been used and closed.
I would also like to note that you can use GTK network manager with KDE. It's not Gnome exclusive. This happens automatically if no other kde front-end is installed.