I created a Windows 10 Virtualbox VM on Windows 10 host. Also installed the Guest additions. The machine runs fine on the Windows host. However, on the Ubuntu 18 host runs ridiculously slow, ya know, can't even use it. Both hosts are running Oracle Virtualbox 5.2. Google reports tons of "Virtualbox slow" cases but none has helped me solve this particular problem scenario. Again, same Windows 10 guest VM, runs fine on Windows 10 but sloow on Ubuntu 18.
I think your problem is not related to your own setup (hw/sw), but more like an issue in Ubuntu/Xubuntu itself. I've also observed a huge slowdown when switching from Ubuntu 16.04 to 18.04 and virtual machines performance too.
I used virtual Windows 10 for testing there along with Ubuntu 18.04 as a host, and I've noticed that it happened after some package updated in Ubuntu. It's definitely not related to the kernel (I even tried the latest Linux kernel 5.0), but some component inside Ubuntu itself. What's more interesting, is that I feel it's related to ... video driver/emulation. No, I'm not kidding, because even my host also became slower.
What I could suggest you would be to compile own video driver in Ubuntu and see the results. I'm a bit old and remember how it looked like in Windows 95/98 when you didn't have video driver installed at all.
Eventually, you may monitor disk and CPU usage in Ubuntu.
iostat command comes with systat package (
sudo apt install sysstat), so if you ran it with
iostat -m 2, it would show you how many bytes are read/written to disk in the host machine.
Additionally, please make sure you're running your CPU at full speed. It's a long story, but you can see your current CPU speed with a package called i7z (
sudo apt install i7z). It will show you whether it's running at full speed (i.e. 2.3GHz or throttling is active and runs at - let's say - 800MHz).