VirtualBox needs “occasional” reboots for best performance

• https://en.wikipedia.org/wiki/VirtualBox

I am running VirtualBox on a MacBook Pro (Mac OS X 10.11.6).

It is running 2 virtual machines:

• Win8.1
• openSUSE

I hardly ever completely shut down the MacBook, I always only pause it.

I “never” shut down the virtual machines, I only pause them as well.

Until I shut down the virtual machines and VirtualBox and also upgraded it today, “this” was going on now for like 2 months. The performance of the guest OSs degraded more and more, the CPUs got eaten up by VirtualBox activity. But after today’s reboot everything is as fine as it always was, when I truly enjoyed it.

But why does the performance degrade that badly? Really, really strange.