POST API calls slowing down as registry size increases

Using COmanage version 1.0.5, I am attempting to import all 67,598 of our users using these POST API calls for each person, in this order:

co_people.json
names.json
identifiers.json
email_addresses.json
org_identities.json
co_org_identity_links.json

The API calls succeed, but as the registry grows in size, they are taking an increasingly longer time. It took only one second to add each person for the first 10 people, but 6 seconds per person for the first 1000 people, and 40 seconds per person for the first 2600 people.

Am I doing something wrong, or is this a bug or an inherent limitation? Is there a better way to batch import data?