Creating shared album takes long time
Files in your cloud count against your storage container and are stored there on the server.

Shared files don’t count against your allocation and are stored in different buckets with different QOS and technical details. 

Depending on if you optimize space on your device and have some items represented as thumbnails instead of the full images (or the image was modified on another device), you would have to round trip all 128 photos down to your device and then back up to the new bucket.

I’ve had preparing hang on one item, so I would cancel that and start with a single image, then add in a few and then maybe do the rest once you’re sure it’s not a server side issue.