I've got an second hand notebook at work with low specs and installed Ubuntu 18.04. The GPU is very old and every effect makes the whole system lag. Is ther any way to reduce UI effects to improve performance?
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.