@koen - My understanding is…
When you have the monitor connected and siwtched ON, then the graphics card is engaged and all should be normally fast when using remote desktop type apps.
When the monitor is connected but switched off the graphics card isn’t used. Which would make sense because in theory the computer doesn’t need to have graphics functions. However when you remote desktop in, it actually does need to perform these functions, so it uses the CPU to do that.
What this device does is trick the computer to use the graphics card all the time, so when you remote desktop in its the graphics card lifting that wieght, not the CPU.
That said, if your monitor is OFF, but you aren’t remote desktopping in, then no graphics functions are needed and there would be no difference in CPU use. This device is really about speeding up the whole remote interface WHILE using remote desktop/VNC/screen sharing.
There’s some more info here…