I recently built new Exchange 2016 servers for a customer and needed to move all of the users' mailboxes to the new servers. When I initially issued the moved requests, the migrations were running about 10Mbps and only moving about 5 mailboxes at a time. There were about 700 users and 1.5TB of email, so obviously this was going to take a very long time I began researching how to increase the speed of mailbox migrations and found several settings that could be changed. Disabling content indexing allowed the migration to occur between 150Mbps and 200Mbps.
Disable content indexing on the target database
Set-MailboxDatabase "DatabaseName" -IndexEnabled:$False
Wait a few minutes for the content index to be disabled.
Verify content indexing is disabled by running Get-MailboxDatabaseCopyStatus
After mailboxes are migrated, the content index will need to be rebuilt
Set-MailboxDatabase "DatabaseName" -IndexEnabled:$True
Invoke-Command -ComputerName GES-EXCH1 {Stop-Service MSExchangeFastSearch; Stop-Service HostControllerService}
Delete old content index file from mailbox database folder
Invoke-Command -ComputerName GES-EXCH1 {Start-Service MSExchangeFastSearch; Start-Service HostControllerService}
Verify content indexing is enabled by running Get-MailboxDatabaseCopyStatus
Mail will need to be reindexed, so the status will be Crawling
Change the number of mailboxes that can be migrated at one time
Edit C:\Program Files\Microsoft\Exchange Server\V15\Bin\MsExchangeMailboxReplication.exe.config
Set the MaxActiveMovesPerSource and Target, and MaxActiveJobsPerSource and Target to desired values
Restart the Microsoft Exchange Mailbox Migration service
Create a new throttling policy
Resource: BitTitan Article - How do I disable the throttling policy on Exchange?