When using Xquartz 2.8.1 (version installed with CalderaRIP version 15.x or newer) and an external high resolution monitor (like a 4k Monitor), it's possible that the display rendering in the Caldera GUI is very slow.
To fix the issue go to the Apple > System Preferences > Displays and select a 60Hz refresh rate on the external resolution display.
You might have to hold the Option key down while clicking "Scaled" to find available resolutions with a 60Hz refresh rate.
The XQuartz behavior may depend on whether it's started before or after an external monitor is connected.
Another but more or less more experimental option is to update to Xquartz 2.8.5.
If this causes issues be prepared to downgrade to version 2.8.1 of Xquartz.