Speeding up Ubuntu guest (23.04) on Windows 11 Home host with VirtualBox

I run VirtualBox v7.0.8 on an updated Windows 11 Home. 

Ubuntu 22.04 simply declined to work once installed on a VM. Even the "guest Additions" CD did not run automatically. Once Guest Addition ran manually, the VM still refused to open reasonably quickly. I tried installing 22.04 twice and then switched to the next in line.

Ubuntu 23.04 simply worked out of the box, but it had issues supporting a 4K monitor. Trying to enlarge the desktop to the full size of a 4K monitor caused blackouts. It was solved by adding more Display memory and increasing general VM RAM. 

Another glitch was that moving a window over a desktop left visual traces - copies of the same window that were living in the background forever. This issue was gone by switching "3D Acceleration" off.

The VBox's VM settings look this way:

KVM Acceleration is selected for Linux guest

Display needs maximum RAM while "3D Acceleration" is better kept off

SATA Controller uses I/O Cache

And the disk is marked as SSD

And yet, switching off "Virtual Machine Platform" feature in Control Panel -> Programs and Features + Restart probably was one single game changer, speeding up the VM a few times.