I have two concerns:
1. How can I speed up the image formation and the reaction speed?
I use guacamole on a Raspberry Pi 2. This computer is obviously not overloaded. The "top" command during a guacamole session indicates that neither memory nor CPU has been exhausted.
RealVNC VNC Server 6.2.1
If the Raspberry Pi 2 is not simply underpowered for this sort of workload (sounds like it's fine seeing as you report neither CPU or memory is appreciably used), I suspect this is due to the nature of VNC on a Mac.
I can't speak to RealVNC in particular, but all VNC servers that I have used on a Mac have been unusably slow, regardless of the speed of the network and regardless of the VNC client in use.
2. How can I work with German keyboard layout?
Things should work without any change in configuration. The client side of Guacamole is independent of keyboard layout, as is VNC.
I have the problem that I do not find the German keyboard layout from the client computer (German keyboard layout) on the remote computer.
Theoretocally, you shouldn't need to change any settings on the remote computer. The remote side will not know that the client is using a German layout, but the keys should still work as intended. Guacamole will forward key events based on key identity, not key location. The VNC server should perform any necessary translation. In fact ... the VNC standard requires the server to do this.