I had a similar issue. I had a calculation in which I made an API request and used a resource with Ash.DataLayer.Simple to convert the data in the response into a list of Ash Resources to have nice Graphql types.
The Resource in question also had a couple of embedded resources and also nested embedded resources. This was also really slow.
The only way I was able to get it to a reasonable timeframe was to do all the conversions myself and create the structs for the embedded resources myself before passing the data to API.bulk_create with the assume_casted?: true option.
