I presume you are speaking for Vista/W7, since W8 and W190 do not have Aero.
Most of Aero runs on your GPU, not taking up CPU cycles, whilst the Classic interface is entirely CPU bound. Aero also makes efficient use of VRAM to speed up even basic desktop redrawing compared to Classic mode.
Most of Aero runs on your GPU, not taking up CPU cycles, whilst the Classic interface is entirely CPU bound. Aero also makes efficient use of VRAM to speed up even basic desktop redrawing compared to Classic mode.