During the migration from a Windows Server 2008 R2 Hyper-V cluster to one based on Windows Server 2012 R2 I have had to migrate a lot of virtual machines. The migration speed was lower than I expected given the available source and destination disk performance. On a 1 Gbps network connection, the migration was only using about 25% of the available bandwidth.
In order to improve the performance I disabled TCP Connection Offload (TOE) on the source hosts. This is because TOE can slow down the VM migration process, which takes place over BITS using HTTPS. The screenshot below shows the setting in the Broadcom software. Having disabled TOE the migration used anything up to 80% of the available bandwidth, significantly improving the migration times.