Reduce interface effects to improve performance on 18.04
I've found out that there's a "Tweak" tool and in the appearence tab is an option where you can turn off animations. Just that made the computer breathe again.
A reasonable improvement you can make is disabling animations, and this can be done without a tool, directly from the command line:
gsettings set org.gnome.desktop.interface enable-animations false
To reverse this, just:
gsettings set org.gnome.desktop.interface enable-animations true
This can also be toggled from gnome-tweaks
referenced in the accepted answer.