You can try using either Scene exporter or Content Gatherer as a workaround for having so many morphs. Those two programs lets you extract every asset being used on a scene. Make another new directory and put all your chars and customization morphs in there(Just data folders, can keep textures.) That fat slow directory will only be added when making new chars for your scene. After making your chars for your scene and saving them as scene/preset, you extract with either one of those 2 programs I mentioned and add the extracted assets as another new directory while temporarily removing the other now unused character/customization fatass directory. Can add/remove that slow directory whenever you're making chars.
It won't take forever to load and be all slow and clunky anymore atleast when posing in your scene. It'll only be slow whenever you use that fat directory to make a new char.