From post-migration review with customer:

[Issues with migration tool]
"Direct image migration slowness. Probably would have disabled if they did this over again. Minutes rather than hours. Some of the ones that were seen should have been small, but they expected the layers to be present on the target side. Took up to 20 minutes to do a single image. Only became noteworthy in a few cases. Some cases did 20 images really quickly, other cases, individual images run for 20+ minutes. Raise an eyebrow, but just waited.

This is an unverified report, but good to look into this and see if it's a consistent problem with the tool or potentially something environmental on their side.