I can use full screen and resize windows, plus other good additions. Obviously yes, or else the rest of the OS wouldn't run smoothly as well. I am not interested in finding a smooth Linux OS on VirtualBox, but rather, to understand and maybe even fix what is going on with Xfce.
Is this similar to the problem in which VirtualBox can't run KDE properly? (I've read that somewhere, and have tried to install openSUSE and Kubuntu)
This actually made it smooth, which means it must be from the Xfwm. There is a final test I performed, where I installed compiz on top of Xfwm. The results are leaning towards the conclusion that the poor performance is caused by XFCE, but this doesn't make any sense to me, as XFCE is supposed to be lightweight and all. However, the movements of the windows still look roughly the same. I wrote a huge loop and let the CPU run at 100%. I found out that moving or resizing isn't really affected by the CPU usage. The CPU usage of host is very small and can't be bothered to get a number. The CPU usage of guest from moving windows is about 10% and resizing is about 2%. The tests I performed, where every OS have the same spec. This seems to only happen on Xfce systems. What I mean by slow is that moving or resizing windows looks very "laggy" (basically just windows jumping from one place to another). I'm having problems getting Xfce systems on VirtualBox to run smoothly.