1.Update the hosts to Server 2012R2

Improved compression and bandwidth usage Windows Server 2012 R2 improves performance on the Windows Server 2012 remote experience, by using codecs that enable better compression, delivering bandwidth savings (for example video content delivery over a WAN utilizes up to 50% less bandwidth compared to Windows Server 2012).

https://technet.microsoft.com/en-us/library/dn283323(v=ws.11).aspx#BKMK_Band

2.Update all clients to RDP Version 8.0 or later it cuts bandwidth usage up to 20%

3.If you server is powerful enough try this GPO:

Administrative TemplatesWindows ComponentsTerminal ServicesTerminal ServerRemote Session Environment“Set compression algorithm for RDP data”