I tested Whonix 11 under Virtualbox just to see the difference, and everything is lightning fast in that version. Scrolling and screen refresh happens instantaneously just like in host system running non-emulated, as one would hope.
Only under KVM is there lag and delay. Even in the initial console/terminal bootup process, under Virtualbox the text displayed scrolls up smoothly so that you can see every line. Under KVM, it is stuttered and only refreshes intermittently, so it's not even related to graphics intensity per se.
One thing to point out is that I am currently testing under Linux Mint, and when I first went to run the Gateway under Virtual Manager, it gave me a message that the package "Spice" could not be found, and thus no graphical display was available. I had to install that package manually in Mint before it would work. Could that have anything to do with it, and could additional configuration be needed? I didn't have to install Spice under Ubuntu, so when I have a chance I will retest under that host and see if I encounter the same sluggishness.