The first script turns off all previews of all layers of all materials of all actors:

for actor in poser.Scene().Actors():
    mats = actor.Materials()
    if not mats:
        continue
    
    for mat in mats:
        for layer in mat.Layers():
            tree = layer.ShaderTree()
            for node in tree.Nodes():
                node.SetPreviewCollapsed(0)
            tree.UpdatePreview()

It's small, but it has such an effect in the Material room: switching to another material is instantaneous now. And of course, here its friend: a script to turn on the preview of only a subset, and only in the current material:

searched_types = [
    'image_map', 
    'ccl_ImageTexture',
    'poser',
    'CyclesSurface',
    'PhysicalSurface'
]

mat = poser.Scene().CurrentMaterial()
if mat:
    tree = mat.ShaderTree()
    for node in [n for n in tree.Nodes() if n.Type() in searched_types]:
        node.SetPreviewCollapsed(1)
    tree.UpdatePreview()

This looks very promising. Thank you, I shall implement both into my app.

The first script was crashing with lights. Here is the updated version:

for actor in poser.Scene().Actors():
    mats = actor.Materials()
    if not mats:
        continue
    
    for mat in mats:
        for layer in mat.Layers():
            try:
                tree = layer.ShaderTree()
            except:
                continue
            for node in tree.Nodes():
                node.SetPreviewCollapsed(0)
            tree.UpdatePreview()

Thanks so much. These will come in handy. I've had to close them all even with Poser 11 since I first got it.