I don't see anything strikingly wrong in your VM setup, except the fact that you're going directly into full screen, I tend to keep my VMs windowed. But, in your guest you have indeed a "problem", and that's the Guest Additions (GAs):
00:00:16.832929 VMMDev: Guest Additions information report: Version 5.2.8 r120774 '5.2.8_KernelUbuntu'
You didn't mention what Ubuntu version you're testing, but my money would be on 18.04, it's the first one that comes with the kernel GAs bundled. That's OK for basic graphics support, but nothing more than that. You need to install the GAs that come with your VirtualBox.
While your VM is not in full-screen, go to the menu "Devices" » "Insert GAs CD image..." and follow the prompts. See if that improves the situation.
A "risky" suggestion would be to increase your CPUs to 2, but since you only have 2 cores in your Mac, you may run into other problems; CPU congestion, host starvation, etc. Try the new GAs first...
