We should try deleting the Incremental Ingest record and allow Photo Mechanic to rebuild it. Please quit Photo Mechanic and the navigate to the following location:
C:\Users\<username>\AppData\Local\Camera Bits, Inc\Photo Mechanic\ingest
In there, you will see a folder called 'incremental'. Delete this folder and test out incremental Ingest again. Does the delay improve over what you reported when testing out similar numbers of files?