I have Mojave 10.14.3, running VirtualBox 6.0.4 and with Debian 9 (with XFCE) as the guest OS.
I have installed the guest additions, no errors. I can verify that the usual vbox modules are running:
vboxvideo vboxsf vboxguest
Everything works fine in Debian, except that the system is extremely slow.
Even the mouse cursor lags when moved around, on a clean system with nothing running. Moving around applications take seconds.
Interestingly, glxgears runs seemingly fine, but again, moving the window itself takes seconds and is extremely sluggish.
It seems like "Enable Nested VT-x/AMD-V" is not possible to select in the options for the guest. The option is grayed out.
I have four CPUs enabled, plenty of RAM. I have enabled 3D acceleration. VirtualBox claims 2D acceleration only works for a Windows guest, so it is disabled.
I have never experienced this slow performance before in VirtualBox, something is clearly wrong, but what is it? And how can I fix it?
This problem is not related to misconfiguration of the number of cores or memory for the guest. I have tried having 1, 2, and 4 cores enabled for the guest. I have tried having 4 GiB and 8 GiB allocated for the guest.
However, if anyone is curious, these are the specifications for my host hardware:
Processor Name: Intel Core i7 Processor Speed: 2,6 GHz Number of Processors: 1 Total Number of Cores: 4 L2 Cache (per Core): 256 KB L3 Cache: 6 MB Memory: 16 GB