Today I was reading How to Install The Latest AMD Radeon Drivers on Ubuntu 18.04 Bionic Beaver Linux wherein I read that “[enabling] DRI3 will increase graphical performance with the AMDGPU drivers”. You can read about DRI3 but the bottom line was to add the following to your /etc/X11/xorg.conf file:

Section "Device"
  Identifier "AMDGPU"
  Driver "amdgpu"
  Option "AccelMethod" "glamor"
  Option "DRI" "3"