I need to add some information. Here is a link to a thread to Jessie downstream distro suffering from issue: https://forums.whonix.org/t/qxl-slowness-fix/1759
Fedora has some patches applied to QXL and maybe they would help: http://pkgs.fedoraproject.org/cgit/rpms/xorg-x11-drv-qxl.git/tree/ http://pkgs.fedoraproject.org/cgit/rpms/xorg-x11-drv-qxl.git/tree/no-surfaces-kms.patch <- this one is interesting!!! http://pkgs.fedoraproject.org/cgit/rpms/xorg-x11-drv-qxl.git/tree/qxl-kms-disable-composite.patch <- maybe of value http://pkgs.fedoraproject.org/cgit/rpms/xorg-x11-drv-qxl.git/tree/0001-worst-hack-of-all-time-to-qxl-driver.patch <- if all else fails
