The RAM impact of mixing apps from different toolkits is real, and measurable, but not quite as bad as the storage footprint issue.

Every toolkit brings its own runtime baggage: When you launch a KDE app inside GNOME, or a GNOME app inside KDE, you're not just launching the app. -You're dragging in its entire ecosystem of background libraries. Typically 80–250 MB of extra RAM is needed the first time you launch an app from the other ecosystem.

Launching a KDE app inside GNOME often also starts:

• kded5
• klauncher
• kioslave processes
• sometimes baloo_file depending on the distro

Launching a GNOME app inside KDE often starts:

• gvfsd (multiple instances)
• dconf-service
• xdg-desktop-portal-gnome

These daemons keep occopying ram even after you close the app. -Adding an additional 50–150 MB depending on which daemons get activated.

GTK and Qt don’t share theme engines, icon caches, font rendering stacks, or accessibility frameworks. So you end up with multiple UI stacks in memory. That's typically another 30–60 MB.

Flatpak, Snap, and AppImage all waste RAM. Some are aware of the disk bloat, but not the ram bloat.

Flatpak apps run inside a sandbox and rely on:

• Flatpak runtimes (GNOME, KDE, Freedesktop)
• xdg-desktop-portal + backend (GTK, KDE, GNOME)
• Bubblewrap sandbox
• D-Bus proxy processes

So, the first Flatpak app you launch loads the entire runtime into memory. That’s ~80–200 MB depending on the runtime. Portals spawn multiple helper processes (GTK portal, KDE portal, file chooser portal, etc). Another 30–60 MB.

You're wasting a total of ~120–260 MB the first time you launch a Flatpak app in a session. After that, additional Flatpak apps reuse the same runtime. (The penalty doesn't stack.)

Snapd daemon runs constantly in the background consuming ~30–80 MB idle, and more when managing updates or mounts. Every Snap app is mounted as a squashfs loop device.
The kernel caches these aggressively: ~50–150 MB of page cache depending on how many snaps you have installed. This cache does count as used RAM (but is reclaimable)

AppArmor + snap sandboxing adds ~10–30 MB per running Snap app.

Total RAM waste for Snaps ~120–250 MB baseline
+ 10–30 MB per running Snap app

-Snap is the only one that wastes RAM even when you're not running any apps.

Each AppImage bundles its own libraries. It loads its own copy of Qt/GTK/etc, and nothing is shared with other apps. Unshared Libraries make it lighter per app. Typically +20–60 MB compared to a native package.

Like with any brand-new install of an OS (without OEM bloat on a recommended spec computer), Linux is going to feel lean and fast. But when you start mixing and matching these different packages, you're not just bloating the footprint, but the ram consumption as well.