Another way to replicate: Simply run the Jessie live CD (debian-live-8.2.0-amd64-xfce-desktop.iso) as a guest in Virt-Manager with QXL driver (default) and try to load Iceweasel and try to scroll through some websites for 10 minutes. Even with spice-vdagent missing, with only the qxl driver the browser is so slow and glitchy it's virtually unusable on my host systems.
Then kill machine and switch to VGA driver, reload VM and try Iceweasel again. VGA appears slower in XFCE at first but then Iceweasel runs great and speed is constant in general. Again there is no spice-vdagent in the live CD so obviously the qxl driver is the problem.
