Did you post this in the Ubuntu forums?
Are both DEs (Plasma and GNOME) installed in the same operating system or are these two seperate systems?
Do they use X11 or Wayland?
How is the memory configured (e.g. vm.swappiness and vm.vfs_cache_pressure)?
Is there a difference in running processes (apart from the DEs)?
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At least my Kubuntu 23.10 (de-snapped, with Kubuntu Backports, 32 GB RAM) and another Kubuntu 23.10 in a VM (with Snaps, without Kubuntu Backports, 4 GB RAM) don’t do that - and there have not been any reports of this behavior in the Kubuntu Forum .Net so far…
Neither does my openSUSE Tumbleweed or my Debian 12 or my TUXEDO OS (all with Plasma 5.27.x and more or less up-to-date system components).
Generally speaking on all of my computers KDE Plasma does use a lot less memory than GNOME does…