The Speed Clock's effects end when night starts, but the Slow Clock's effects do not. As this has an effect on the spawns of hostile mobs, I believe this unintentional.
I'm not sure if this bug still exists in 0.10.2; I'm currently not in a position to be able to check.
I'm not sure if this bug still exists in 0.10.2; I'm currently not in a position to be able to check.