On-prem FileShare to ODFB migration

Hello there,

We tried to migrate our on-prem fileShare drives to ODFB for a total of 1800 users using Migration Manager tool from the SPO admin console. It was very slow. In fact, it only migrated about 500 users within 48 hours window. The number of files were approximately 180K only. There is not even a single file with more than a GB file size. In the end, it was determined that by increasing the number of threads will speed up the migration. Sure enough it worked. Question: How is the process of increasing number of threads done? We only have 3 machines performing the migration and nothing was changed from our end so I assume it's something done maybe in the backend configuration for SharePoint online with PowerShell maybe??? Any thoughts on this? Let me know please.  Thanks and have a great day.