Canelo isn't fast himself.
GGG obviously has very heavy hands. What he may lack in hand speed is made up in his technique and ability to cut off the ring. I don't see anyone in boxing right now cut off the ring as effortlessly as Golovkin. He can literally cut off half the ring in just a couple strides and he's smart enough to know the angle he wants his opponent, i. e he's thinking about what moves he's going to make before making them in order to trap his opponent. His footwork is top notch and you'll see the tricks he does with his feet to get his opponents out of position.
A boxer puncher like Canelo isn't going to be able to out box him, he's going to have to goto war and that heavily favors GGG.
The best way to handle GGG is to smother him. TThey'd basically have to try to Andre Ward him (smother, clinch, hold) and pray to Allah he doesn't catch you.