The two causes are RDP plug-ins and UDP usage under certain conditions.
This one is quite easy to test out. You simply go in the Experience tab of your RDP sessions and uncheck the Load plug-ins in embedded mode option. Do this first as it has a really low impact.
RDP Experience Tab
There's an option in RDM to disable UDP usage. To do so, open Remote Desktop Manager as an administrator and click on Tools -> Local RDP/RemoteApp Manager and click on Disable to disable the UDP settings.
This one is a bit trickier, some search results hint that this is caused only when going through a VPN, others mention that it occurs only on Windows 8 paired with a Gateway server, but this solution has worked for many of our users, so we suggest you try it.
It requires a registry change, so all the usual warnings and caveats apply here, back it up first! Consult this Microsoft support article for more information
The fix is to disable UDP on the client workstation using these simple steps:
1.Navigate to HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Terminal Services\Client.
2.Create a DWORD named fClientDisableUDP and assign it a value of 1.