I have spent many an hour trying to figure out why VirtualBox 6 has given me terrible graphical performance on Linux Guests. I’m going to give you a no-mess solution that just might save your sanity.
Within the Settings for Your Virtual Machine, navigate to the Display > Screen tab.
Under the Graphics Controller dropdown, select VMSVGA for your Linux guest.
Next, you will need to boot into the virtual machine and reinstall the VBOX Guest Additions for this to work (and cement it in with the obligatory reboot)
So you may be wondering why or when you might need to use this guide?
You may have upgraded to VB6 from VB5 or earlier and trying to use a pre-existing VM.
You may have installed VB6 for the first time, created and installed a new Linux guest VM and experienced graphical slowness.
You may have tried (mostly) every setting within VBOX and viewed every forum post on the internet regarding VB performance issues and have got no further.
Admittedly, I was incredibly close to giving up altogether with VBOX 6 and going back to VBOX 5 as I have been struggling with this since VBOX 6 release. Finally, I have my VM back =)