Ways to reduce load time for item/organization assignments

Summary: Looking for ways to reduce load time to assign 55 Orgs to ~50K items in PIM.

Content (required):

We have ruled out the options of using FBDI org or Import maps as it would require loading 55 items , one for each Org for 50K items.

We tested in Non-Prod instance by updating Pack Type for items through Item Batch using import maps. Updating the Pack Type triggers Org assignment rule which assigns 55 orgs to items. We ran the item batch for 6K Style Items first and it took 2hrs to complete ~50 items per minute.

However, for SKUs , the performance decreased drastically. There are 29K SKUs which we are loading in 3 batches of 10k each. First batch took 6-7hrs , second batch took 12-13hrs, final batch took 3hrs 30mins to complete . With item batch and assignment rule we are not able to predict the performance.