I'm using xrdp w/ KDE and these compositor settings appear to have helped quite a bit:
scale method: crisp
rendering backend: OpenGL 2.0
Tearing prevention: Only when cheap
Keep window thumbnail: never
Allow options to block composting: yes
Edit: To update for clarity, these values are in KDE's settings manager under
Desktop Effects -> Compositor. They are not client-specific. The example is modifying the server's desktop environment settings to "dumb them down" a little bit.
Since XRDP is delivering the server's desktop via RDP protocol to your client, if XRDP has less data to send it follows that the remote desktop should be more responsive.
There was a question about clients - As far as clients I was using at the time, I'm sure it was KRDC and windows RDP client. I can't recall modifying any default settings on the clients, other than in KRDC making the window full-screen and client native resolution (so I could make it a separate virtual desktop).
Also, there is another response about reducing / eliminating default encryption from the RDP stream should be very helpful in making the entire process less CPU-intensive from end to end, if that's something your environment will allow.
Hope this helps.