Originally Posted by DavidN
Layers in themselves don't cause slowdown - I have an options frame with about twelve or so, as they're a useful way to separate out multiple dialogue boxes.
But their use combined with scrolling can cause a lot of slowdown because of the way that the collision masks work in MMF - the first layer has a complete collision mask calculated before the frame starts, and the ones above it are tested object-by-object. This means that if the majority of your gameplay happens on a layer higher than 1, it'll tend to be slower - often unnoticeably but sometimes a lot slower, if there are a lot of things going on.