In the manual it says this:
[quote:3m4pwoqi]Effects are visually impressive, but can slow down the game's performance considerably if over-used, especially on mobile. Using an effect on objects with many instances is especially slow - often it is much more efficient to place all the objects with the effect on their own layer, then apply the effect to that layer instead. This allows the effect to process everything in one go, rather than having to inefficiently re-run the effect for a small object over and over again.
So, if you put the effect on the layer and not on the object, i think it doesn't matter how many objects are on the layer.