External monitor slow/laggy when in standlone mode

Recently, I installed an external GPU (HD6870) on my laptop which connects to an external monitor. Every time that I extend desktops, the graphics work just fine. The problem arises when I try to disable the built-in display of my laptop. The screen becomes laggy and, when watching a video, the quality is awful with motion.

I only have this issue with ubuntu (Windows works flawlessly). I tried installing the fglrx drivers but they are not supported on 16.04. The open-source radeon drivers come pre-installed in ubuntu.

Here are the contents of /var/log/Xorg.0.log after I enable the built-in display:

[ 2159.566] (II) intel(0): resizing framebuffer to 3046x1050
[ 2159.574] (II) intel(0): switch to mode 1366x768@60.0 on eDP1 using pipe 0, position (1680, 0), rotation normal, reflection none
[ 2160.064] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz
[ 2160.175] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz
[ 2160.271] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz
[ 2160.375] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz
[ 2160.487] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz
[ 2160.591] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz
[ 2160.692] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz

Here are the contents of /var/log/Xorg.0.log after I disable the built-in display:

[ 2249.382] (II) intel(0): resizing framebuffer to 1680x1050
[ 2250.252] (--) RADEON(G0): HDMI max TMDS frequency 225000KHz

2 Answers

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!

My solution is make prime-select use nvidia not on-damand. Run sudo prime-select nvidia to do this.