Hi, I have been facing similar issues in my environment where RDP to Win2k8R2 servers was working fine, however RDP to Windows Server 2012 servers was very slow and sluggish. After some research came across some group policy settings which resolved the issue for me.
1. Go to Computer Configuration > Policies > Admin Template > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Connections
2. Select Network Detection on the server
3. Enable and Select the Network Detection level to "Use both Connect Time detect and continous network detect"
4. Slect RDP transport protocol and Enable
5. Select Transport type = Use only TCP
Please do let me know if this resolves your issue
