Slow File Transfer with IPSec Site to Site VPN to Azure VM

I'm seeing about 2 Mbit/s throughput on a connection that should be able to get 25Mbit/s.

This is a simple scenario with one Windows Server 2012 R2 VM copying data over SMB2 to another Windows Server 2012 R2 VM in Azure over the Site to Site VPN. When I look at Wireshark, I see quite a few TCP Retransmissions and TCP Dup Acks.

I've tried setting TCP MSS Clamping to a number of different values, but none seem to work. Packet length size is 1414 (according to Wireshark). If I try to ping with a packet length of 1414, it fails, but 1410 works. Below is a snip of what I see in Wireshark. When I set the TCP MSS Clamping value, what should this be set to, and should I also set the MTU values on each of my servers (they have the default of 1500).