Sluggish window movement with a Radeon GPU

Problem description

When I move around or resize windows, or do similar actions, the movement behaves very sluggish, i.e. the visuals lag behind where the mouse has moved before. Due to this problem, browsing in Firefox is very uncomfortable unless I force-enable the hardware acceleration, which causes other less-noticeable problems.

  • When moving a window: It lags only when I move the window quickly. When I move it slowly, it works smoothly.
  • In Firefox scrolling is very laggy, except when I enable the experimental layers.acceleration.force-enabled setting in about:config, which makes the scrolling smooth. In some other applications, for example Chromium and Geany, the scrolling and other window actions work smoothly without problems.
  • When I start glxgears and move the window around, the framerates printed in the terminal drop severely during the laggy window movement. This happens with and without vsync in glxgears (vblank_mode=0 glxgears); for example, the framerate of glxgears with disabled vsync can drop from about 5500 to just 35.
  • There is no high CPU usage when it is lagging and the graphics driver is loaded and works for e.g. OpenGL rendering in games.
  • For some reason with another PC which has an intel GPU and the same Lubuntu version, the problem does not happen.
  • The problem also did not happen in Lubuntu 18.04. I first noticed it after updating to 18.10. With a fresh installation of Lubuntu 20.04 the problem still happens.
  • When I move around a window, the thing behind the window matters for the problem. The movement is laggy when I move the window around while it is above the desktop background. However, the movement is smooth when I have a big glxgears window in the background and move around a small window in front of it. As soon as the small window is no longer on top of the big glxgears window, the movement is laggy. A big Geany window in the background has the same effect, but a big Pcmanfm window does not.
  • I have tried the awesome window manager, compton with various parameters and custom radeon driver configurations, but I could not find configuration parameters where the problem does not happen.
  • The problem did not happen when I used the PC remotely with a TigerVNC client. If I remember correctly, there was no vsync with VNC.

Information to include

Version of Lubuntu

18.10 and 20.04 but not 18.04 (see above)

Hardware Information

The output of inxi -v 5: https://gist.githubusercontent.com/FHof/182e2a83ebbb51314978362e69cecbba/raw/183b174826eab194670cca57e275e92e54ffa8ef/inxi_output.txt

Logfiles

The Xorg.0.log file: https://gist.githubusercontent.com/FHof/8ac3ed7ff53f6431b6e3b94f037b6610/raw/cb302877e32d17a2e3b158f7c0e964b51412f2eb/Xorg.0.log

I don’t see interesting information in the syslog file, but I can upload it if you explicitly ask for it.

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