I bought a mini-pc to use it as a server, but I need to have the GUI enabled so I can use some software on an external screen. The problem comes when I unplug the HDMI from the PC / I turn on the PC with the HDMI unplugged. The GUI is so slow that it is unusable (I'm connecting through VNC). The problem becomes solved when I plug the HDMI cable at any time. If I unplug it again, the lag comes back.
I'm using Manjaro Xfce, and the VNC Server / Viewer is working correctly. I'm using an 8th gen i5 processor with an integrated graphic card.