How to install KDE Plasma without installing full Kubuntu Desktop?
As DK Bose said, I tried executing this command:
~$ sudo apt-get install plasma-desktop
And voila, it worked! It certainly does. It neither installed full kde-desktop, nor installed any more annoying apps nor modified my bootup configuration. I'm now using KDE without any problems, and the best part is my PC doesn't slow down.
Thanks to DK Bose and the Ask Ubuntu community who put efforts to solve my problem. I appreciate their efforts.