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Having been around KDE and GNOME for some time now and ran these DEs side by side, I must say that rival DEs such as GNOME do run a bit more RAM friendly. The last really big uptime was ~4 Days. At that time I shutdown my session because my RAM was filled up to 4GB. Guess which 2 processes had most of it? Exactly Amarok and plasma-desktop. Well yes I do have 8GB of RAM in my machine but I know people who don't have so much in their boxes and still do want to have a pretty system like mine.

I remember that some time ago GNOME wanted to supress its startup time to as little as possible and thus get rid of the splash screen.
They succeeded and I couldn't recommend people with a slow machine to use a KDE-Desktop. Its just too hungry to use on a 1GB RAM EEEPC.

So why dont we try and do something about it? From top to bottom.
Report every single KDE application that eats your RAM.

What do you think?

Actually, the RAM should be pretty much filled up at any given time no matter which DE you use. Memory is wasted if it's not used, essentially.

Next time you think memory is being wasted, please check the output of the free command. Chances are that a lot of the "waste" is actually disk-caches and buffers. Which speeds up the operation of the system.

That being said, of course some programs can be improved when it comes to how much RAM they use but personally I can't say I notice much of a difference, if any, between Gnome and KDE with equivalent programs running.

You say that your KDE setting uses 4GB out of 8GB. Well my uses cca. 1GB out of 2GB. As Kryten said that's how RAM is used due to cache.
I have Windows in VirualBox which I use for Photoshop and InDesignh, which are both known as RAM heavy apps.
Well you'd expect that given my KDE uses 1GB by default I would have a lot of problems running them, but actually I have no problems what so ever. Because the cache is cleared and after I stop VirtualBox my RAM comsumption falls from 1GB before I started it, to ~600-700MB after it's used, due to cleared cache.
It should be more or less the same with your KDE, if not then something is wrong with your install.

Idea is invalid.