Prior releases of Ubuntu would allow the user to choose a 2D mode. With the modernization of Ubuntu Unity this has gone away.
The alternative is now to set a Low Graphics mode using Compiz Settings Manager.
Here are the steps to do this.
sudo apt install compizconfig-settings-manager
Open compizconfig-settings-manager
Click “Ubuntu Unity Plugin”
Scroll to the bottom of the General tab and ensure that Enable Low Graphics Mode is checked.