First, set Paravisualization Interface as KVM.
Make sure you allocate enough CPU and RAM. If you’re running Desktop Environment, you should allocate at least 2G RAM. 4G RAM recommended.
If you meet any graphic problems, turn off 3D Acceleration. It will solve most graphic problems.
If you find disk I/O is very slow, go to Storage setting page, click “Controller: SATA”, then check “Use Host I/O Cache”. You will find your Virtual Machine run fast beyond your imagination, even thought the disk is on HDD, not SSD. So you should enable this before installing Operating System, it will speed up your installation. (VirtualBox disable this by default)
Set sound hardware as AC 97, not Intel HD, to get best compatibility. Otherwise, you will meet some strange problems while playing sound, especially laggy or stuck.
Finally, don’t forget to install the VirtualBox Guest Additions, it will give you full power. Instructions here: https://www.virtualbox.org/manual/ch04.html#additions-linux
Last thing last, remember Snapshot will record the hardware settings too. After restoring snapshot, you should set your hardware settings again.