Installed Ubuntu minimal and found that the memory usage is high in idle state, 1.2GB! What takes so much memory just to run a DE?
Is this GNOME? If so you should probably file a bug against GNOME 
Although you may be affected and may just want to mark yourself as affected by this bug, as per the video in this article, the memory usage increases every time you do some action in GNOME Shell.
Of course, it is Gnome. Installed the minimal to just to see how it works. I am not affected by this “bug.” Just reporting.
I intend to post status updates on memory usage fixes in:
For specific issues you can also subscribe to these:
https://gitlab.gnome.org/GNOME/gnome-shell/issues/160
https://gitlab.gnome.org/GNOME/gnome-shell/issues/159
But I imagine there will be more fixes in future.
Now, in idle state, the memory usage had grown up to 1.3GB. I had opened Nautilus and closed it, but the memory usage had not gone back to 1.2GB, but stayed at 1.3GB.
I agree, Ubuntu lost its purpose of fastest OS with Ubuntu 18.04 beaver update. It makes your system very slow and unusable.
Other than those shell issues, If you have more memory os try to pre-allocate more. And Kernel (>4.2) also tries to pre-allocate memory as well. For example Unity with kernel 4.2 (or a patched version of kernel) only consumes 500-600 Mb on boot. After fixing several memory tweaks it reduces to 400 MB.