It's a custom scen, cs players are used to fast (even though it's rm style). The map doesn't require as much micro as regular games or if so it's mostly in grush which isn't too hard fast speed anyway. It has a very slow start so it would be boring to start in normal speed, and it's also generally a map that has 2 hour + games often, and really I think fast speed is good for post imp because otherwise it can get boring. Obviously difficulties can arise when you're mixing land and water for micro during castle age but I think for the majority of the game fast speed is the optimum.