Explanation
The migration is causing the slow performance. This suggests that the on-premise Exchange server is struggling under the load of copying the mailboxes to Exchange Online. You can reduce the load on the on-premise server by reducing the maximum number of concurrent mailbox migrations. Migrating just a few mailboxes at a time will have less of a performance impact than migrating many mailboxes concurrently.
Reference:
https://support.microsoft.com/en-gb/help/2797784/how-to-manage-the-maximum-concurrent-migration-batches-