External monitor slow/laggy when in standlone mode

I encountered the same problem (Ubuntu 18) and could solve it by selected the 'proprietary, tested' driver for my graphic card!
In other words, the recommended one.

You should check if you are using the 'official' graphic driver or another one / old one like this:

• Open the "Software & Update" app
• Click on the "Additional Drivers" tab and wait a bit for it to load
• Make sure to select the driver with the "(proprietary, tested)" text!
• Click on the "Apply changes"
• Restart computer

In my case, I was using the "X.Org X Server - Nouveau" driver (I can't remember how/when I selected this one..)

I switched to "NVIDIA driver 440 (proprietary, tested)" an rebooted my PC: no more issue / lags when using external screen only!