Comment 3 for bug 1316035

Actually it seems that the cause is the new xserver-xorg-core, it seems that XAA rendering was ripped out from 1.15.1 (no libxaa.so in package).

In old Xorg.0.log rendering was falling back to XAA:
[ 4.696] (II) MGA(0): Using XFree86 Acceleration Architecture (XAA)
[ 4.696] Screen to screen bit blits
[ 4.696] Solid filled rectangles
[ 4.696] Solid filled trapezoids
[ 4.696] 8x8 mono pattern filled rectangles
[ 4.696] 8x8 mono pattern filled trapezoids
[ 4.696] Indirect CPU to Screen color expansion
[ 4.696] Screen to Screen color expansion
[ 4.696] Solid Lines
[ 4.696] Dashed Lines
[ 4.696] Scanline Image Writes
[ 4.697] Setting up tile and stipple cache:
[ 4.697] 32 128x128 slots
[ 4.697] 22 256x256 slots
[ 4.697] (==) MGA(0): Backing store disabled
[ 4.697] (==) MGA(0): Silken mouse enabled
[ 4.698] (==) MGA(0): DPMS enabled
[ 4.698] (WW) MGA(0): Direct rendering disabled
[ 4.698] (==) RandR enabled
[ 4.707] (II) AIGLX: Screen 0 is not DRI2 capable
[ 4.707] (II) AIGLX: Screen 0 is not DRI capable
[ 4.800] (II) AIGLX: Loaded and initialized swrast
[ 4.800] (II) GLX: Initialized DRISWRAST GL provider for screen 0

In 14.04 there is no XAA acceleration anymore
[ 8233.057] (EE) MGA(0): [drm] Direct rendering only supported with G200/G400/G450/G550.
[ 8233.057] (II) MGA(0): Using 3289 lines for offscreen memory.
[ 8233.057] (==) MGA(0): Backing store enabled
[ 8233.057] (==) MGA(0): Silken mouse enabled
[ 8233.057] (==) MGA(0): DPMS enabled
[ 8233.057] (WW) MGA(0): Direct rendering disabled
[ 8233.057] (==) RandR enabled
[ 8233.062] (II) SELinux: Disabled on system
[ 8233.062] (II) AIGLX: Screen 0 is not DRI2 capable
[ 8233.062] (EE) AIGLX: reverting to software rendering
[ 8233.074] (II) AIGLX: Loaded and initialized swrast
[ 8233.074] (II) GLX: Initialized DRISWRAST GL provider for screen 0

unfortunately without XAA moving a window is like a slug