After some investigating this seems to be an issue with the Hue V2 API after some bridge update. Hue Essentials migrated to this API in the past (New color display is terrible - #3 by Thomas).
It seems when you set the light to a color temperature, it is briefly reported in the new API, but the value then reverts to a color value which is invalid. The color temperature value cannot be read anymore by Hue Essentials. Unfortunately this is not an easy fix as the Hue bridge reports the wrong value, so Philips Hue would need to fix this in their bridge API.
The performance issue for the initial loading of groups will be fixed in the next update of the app, version 4.0.0. I expect it to be available in the next week.