Passthrough Blendmode Cost

Hello, I tried to search for this, but came up emptyhanded. I am wondering the compuational cost of the Passthrough blend mode. Basically, If I have a group that has a bunch of layers in it and set the master group blendmode to Passthrough, does that add complexity to the compuations of the layerstack and would that start to slow down the scene. Even if it was a little slowdown, as I added to that layerstack, or the more groups I added like it with the Passthrough blendmode enabled, would this also add more cost to the scene, slowing down performace, versus if the blendmode of the of the groups was set to the default Normal blendmode. Or is the Passthrough viewed as more of a non-entity container? My goal is the optimize performace on complex scenes and not add cost when unnecessary, although the Passthrough blendmode is necessary sometimes. 

I.e. does Passthrough add cost veruss Normal blendmode, and if so, at what point would it become a problem. Thanks!