Like most thing in Windows, the sauce is buried under layers of Registry tweaks and/or Group Policies, but enabling RemoteFX (released with Windows Server 2008 R2 SP1) for RDP enables both h.264/h.265 stream compression, and with additional settings changes to enforce usage of the GPU on the desktop session over RDP, you can get solid 3D acceleration. In this video from Wendell @ Level1Techs, he had 31 virtual desktop instances all running with full GPU acceleration, with RDP as the VDI client software. https://www.youtube.com/watch?v=tLK_i-TQ3kQ. As another data point, I had game developers remoting into their workstations during Quarantine, using RDP with full graphics acceleration using the same tweaks.
What the shinier VDI/Remote Desktop solutions like Sunshine+Moonlight, Parsec, Horizon, Citrix do for you is make all of this easier to set up and use. But as far as capability is concerned, RDP is quite powerful and quite capable, assuming you jump through the hoops to set it up right.
