Ok first, like I asked is this all of a sudden or has it always been this way? Second, does it act this same way on several different cameras.
I think I remember you said it has always been this slow. So now you need to get another camera and try it. If it is the same it is the lens not the camera. Let's suppose it is the lens. Go outside on a nice day and try it. Good contrast so the AF has all ther info it needs. Is it doing its thing? Poor light will slow even the fastest lens. If it works as it should the lens is probably OK.
Lastly has the lens ever gotten wet or been dropped? If no, the lens is probably OK. if it did see some rough use send it to Canon for a C&C.
EB
EOS 1DX and 1D Mk IV and less lenses then before!
