Recently, I found it became very slow when I am logging onto my development VM, a windows server 2012R2 instance through Remote Desktop. There are many kinds of solutions in Google's search results. I tried many of them and found only this one suits me.
On the Windows Server 2012 machine, disable the Large Send Offload via the following steps: Open Network Connections. Click Change adapter settings Right-click the icon of the Network card and select Properties. In Networking tab, click Configure… button. In the next window, switch to Advanced tab. Click the Large Send Offload Version 2 (IPv4) and change the value to Disabled. Click the Large Send Offload Version 2 (IPv6) and change the value to Disabled.
Your RDP connection will disconnect right away after you apply the change. Don't worry, connect it back and you will find the annoying delay disappears!