Here is a quote from Ashleys article on Performance Tips:
[quote:20qhpj29]Number of layers usually has no effect, unless most of the layers have changed their opacity, have effects applied, have a blend mode other than 'Normal', or use 'force own texture'. However typically there is no performance impact from using multiple layers with default settings.
So if you are really seeing such a performance drop, it might be worthwhile posting a minimal bug report showing the before and after .capx.