Gnome ToolKit and Gnome Developer Kit= GTK and GDK respectively.
In Linux (ignoring BSD), you generally come down to Two Main Toolkits.
Gnome Toolkit GTK and Qt (Pronounced Cute); Qt doesn't really stand for anything... The nearest you can get is Q Toolkit.
They are very different.
KDE (Kool DEsktop snicker)is based on Qt (QT) and Gnome is based... well, obviously... on GTK.
GTK Themes will not really work on KDE.
They are for GTK. On KDE, you must look for KDE Themes. There are many places you can search, like kde-look to find some to your liking. Search Plasma 5 themes
There is a LOT you can do and a lot of customization available in settings. But KDE is different from GTK in another regard:
Where you can easily go into the source or the files in GTK in order to customize, without ever needing to touch a Settings App, in KDE, you are limited almost to only using the settings applications. And while they offer a lot, what they offer is all you get.
For most people, this is enough. It is... nowhere near enough for me...
For you as a student, whose primary goal is a performance working computer while you focus on your other studies, KDE is a Fine Choice.
Just keep in mind that when using it, you are better served sticking to thigns formulated for Plasma Desktop, not Gnome Desktop. It's really almost like using a different Operating System. Like the difference between Windows and Linux, really...
@AdarshMeher these are very good questions. I suspected Gnome had a performance edge on Plasma. I admit, it seems greater than I suspected.
I'd gladly help; If I knew how. I am woefully unqualified to help on KDE/Plasma.
But your posts stand - hopefully others can help you. And if not...
Hopefully you enjoyed the outing on Plasma desktop and enjoyed seeing the sights.
If it comes to the wire; Remember the tip about using Synaptic posted above.
Perhaps it is loading too many things.
Can you try launching System Settings > Startup and Shutdown > Background Services and disable any unneeded process?
I use gnome and only gnome. Suits my needs and work style best. When the switch to gnome 40 or 41 comes I will, sadly, say farewell to Zorin. I simply do not want to be without workspaces-to-dock. That will still leave me with Ubuntu 20.04 LTS and Debian 11. With luck, Debian 11 should be around for nearly five years. As I'm 86 that may suffice.
I also run EndeavourOs and Fedora 35 out of curiosity, but I don't really use them. For now Zorin 16 is my daily driver.
As Zorin OS 16 is based on Ubuntu 20.04, it will continue to receive the same updates for the same length of time. So you may not need to say farewell to Zorin OS.
