Attack Speed Reduction should affect your base attack speed.
Currently, all attack speed buffs are added together and then modify your base attack speed. For example, 3 items giving you +25% attack speed will add together to increase your base attack speed by 75%. You also get around 1.5% attack speed per level, which would be added to the +75%.
Attack Speed reductions are simply subtracted from your attack speed buffs. A +30% Attack Speed increase and a -30% Attack Speed reduction will simply cancel out. It is possible for the number to go negative if you haven't built any attack speed.
The end result of this system is that Attack Speed reductions are most effective against gods that have not built attack speed, which are also the gods least likely to be getting most of their damage from basic attacks. Meanwhile, the gods whose attack speed you want to nerf the most are the gods who are the least affected. Someone getting +200% Attack Speed from items and abilities is not really going to notice if it drops to +160%, since their number of attacks per second has simply dropped from 3.00 to 2.60 (roughly), a 13% decrease. Meanwhile, a level 20 god with no Attack Speed built is going to have their Attack Speed drop from +28% to -12%, going from 1.28 to 0.88 attacks per second, a 31% decrease.
It really doesn't make much sense for it to work this way since it makes Attack Speed reduction good early game and against gods not building Attack Speed, but falls off pretty hard late game. If instead the base Attack Speed was reduced by 15 or 20 percent, this would have a consistent impact at all stages of the game. It would also mean that simply building more Attack Speed would not completely counter attack speed reduction.