MAX AB, I believe SID are designed to a standard gradient, (I can't remember what that is off hand tho 3.3% seems to stir some neurones some where). So a speed constraint will usually be an ATC tactical constraint. If the published SID has a higher sid gradient published eg Hongkong, then it is fair to assume it is due to obstacles. In some places where you have a higher gradient, eg Warsaw our company states that the higher gradient is due to ATC constraints. Cheers