Which would be "faster" more "responsive" virtual machine software? GNOME-Boxes or VirtualBox?
GNOME Boxes is a graphical user interface similar to virt-manager aimed to access virtual machines based on Qemu or for viewing remote desktops:
That said, Boxes will be sharing a lot of code-base with virt-manager project, mainly in the form of libvirt, libosinfo (not yet used by virt-manager) and qemu.GNOME
Therefore it is not expected that there will be any performance improvements in comparison with existing virtualization software listed here. It may however be easier to access you virtual machines from GNOME. There is also a question on this topic here.
From my experience Virtual Box is so easy to handle that it does not really need any user interface other than the one provided. It also has the advantage of a being a cross- platform application.
Although speed is a relevant question, I think what would be more important is how it handles processor threads and memory allocation.
I'm newish to Xenserver and definitely no experience with Gnome-Boxes (yet), but I've been using VirtualBox and VMWare for a few years now. VMWare V-Sphere (I believe this was only available with an Enterprise license) has the nice bonus of being able to throttle up/down processors and cores and dynamic ram allocation - meaning if another VM requires more power or memory, it has the ability of doing so without going below certain thresholds that may impact performance overall.
VirtualBox - does not have this feature from my experience. Granted you can set a "Maximum" throttle for the CPU use, and the amount of virtual cores to use, but I don't believe I've seen settings to dynamically allocate memory (Perhaps the memory allocation is a maximum and it throttles it down/up in the background if needed?)
I'm sure XenServer and the Xen Project have these features built in, but I'd be curious to know how Gnome-Boxes handles it, or if it handles these situations better than VirtualBox. I think I'm going to throw a system together to test this unless someone has this information already :-)