Hi, I had the same problem, but its easy to fix it in SDF Modeler itself: you simply go to layer panel, to select each one but in the inspector panel in the right view, you see layer name and just under the resolution with the arrow you can toggle between resolutions, by default it's 512*512*512, but you can lower these values like 128*128*128 or even lower, but evidently you will loose in quality, so be aware of it ! If you have different layers, each one can have its own resolution, so you can tweak different resolutions accordingly, if more intricate details exists, do not go too low... if the shape looks simpler, you can get relatively low.
In the Blender side, applying a decimate modifier after this is sometimes not necessary, and the make normals outside works perfectly.
Hope it helps. Cheers. Dan