Not all weight gainers digest slowly, I am currently taking IDS Smart Gainer for my post workout and it has been helping my recovery so much, and it is both fast digesting protein and fast digesting carbs. Weight Gainers are great Post workout as long as long as they are fast digesting, now BSN Trumass and Cytosport
Cytogainer are both slow digesting. As far as pre workout, it all depends because I take supplements before workout and generally they are to be taken on an empty stomach, but I do take a fast digesting 1 hour before I take my supplements.