Speed up Integrations that use REST API

Summary:

Is there a way to speed up REST api roundtrips to Fusion?

Content (required):

We have an integration that processes an inbound CSV file to update costs in inventory. For each line, the integration needs to call multiple REST APIs to update data in ERP. Currently, this takes about 7 hours to finish about 8000 lines.

However, my question is more generic about designing such integrations. How do you design an integration that will process this faster? Is there a way to run multiple (inbound) lines in parallel? I understand our OIC server is load balanced etc. etc. How does one leverage that to speed up processing?