After every implement the Plasma Workspace becomes heavier and my old netbook runs now with it very very slow.
Because the question: Do the current PIM-Suite and other Software from KDE-5.x without Plasma?

- There is no such thing as KDE-5.x
- Plasma does not consist of applications, it is the desktop
- The applications have always worked independent of your desktop choice

I must say that I cannot confirm your observation. Plasma-5 did run excellent on my super old Thinkpad X200s, better and more responsive than Plasma-4 did. Plasma-5 is not 'heavy' at all.