spice used page dirty tracking only to figure which screen areas need updating (with qxl in vga mode).
update areas are basically tracked with scanline granularity, resulting in bulky screen updates being sent even in case only a small portion of the screen has been updated.
Keep a copy of the most recent screen content send to the spice client, use it to figure which screen areas need updating instead if just sending the whole scanline.
Required bandwith goes down, interactive performance goes up. Especially noticable with win8 guests which have no qxl drivers (yet) and thus run in vga mode all the time.