...depends somewhat on distro, but I don't know of any way of doing this from the command line....
For some distros, on the first occasion that you run kde as a particular user, there is a dialogue box that pops up asking you whether you want all of the eye candy (kandy?) or whether you want speed, and gives you a slider to choose between the two. This is nice, but I can't spot it as a runnable applet here on this kubuntu box, so I am assuming that it isn't generally available after that first run (could be wrong, though, maybe its just kubuntu which is missing this).
Given that, as far as I am aware, all this does is package up the various desktop effects and give you one easy slider to control them, you could go into to the contro centre appearance and themes/desktop settings and change them individually to suit yout taste/hardware performance.