This is as a result of Microsoft's default connection speeds:
Observed single MAPI migration throughput is in the 0.1-0.5 GB/hour range. More concurrent migrations can be used to achieve higher data migration rates. Single MAPI migration throughput will slow down when on-premises servers or the network is at capacity.
To resolve this issue. run concurrent migrations to improve/increase throughput.
See other KB articles on how to calculate the number of connections you can configure.