After clean install and configuration, you can also use clonezilla if you will to test something extreme. It took less time than reinstall from zero, at least if you use a lot of customization.
I use a lot of kde software in my zorin lite, (e.g. Dolphin as file manager), but I haven't notice an abnormal memory usage. Maybe you have some service running in background, like Network Manager or Kwallet.
In this guide , at the bottom, there is a way to stop loading such services.
That's is the reason why LTS release exist. After you have configured your system you can maintain it for at least 4-5 years .
About D.E., let's start a flame 
(I'm just kidding)