It isn't the object count, it's the effects! It sounds to me like you put the black effect on each of the objects and then placed them around. So it has to calculate that effect for each object every tick. This is horribly inefficient and would slow down any computer. I have far more objects than you are describing in my scenes with shadow sprites and it has essentially no impact. But I'm also not using effects to achieve it.
There are two solutions that will drastically improve performance. The first, which is the better option, is to make separate shadow sprites for your objects, place them on a shadow layer, lower that layer's opacity. Import your shadow sprites at 100% opacity and then lower the shadow layer's opacity so they blend with each other. Effects, especially blur, are really expensive to run. You should really only be using them for more dynamic elements. Ie, use blur only on an object where the blur needs to change or go away dynamically.
The other, which is a kind of middle ground, is to put all of your objects on the layer and then apply your black effect to the layer, not the objects. This way, it processes the effect once over a larger area instead of potentially 500-600 times with how your were describing. I would go with the other option I described and pre-bake as much as possible, though. I have far more than 1,000 objects on my layouts with no ill effect, and my computer is far weaker than yours.