Hi Patrick,
While Homey sends all commands at once (with any protocol, not zigbee in particular), asynchronous, the Group app should send commands to group members one by one, synchronous.
That tiny delay in between each command always was present, and, in previous app versions, it was adjustable.
I couldn’t find that setting either, in the current app version, with Light groups that is:
I just found out it’s present in a United group advanced settings tab:
Can you confirm the lights act as expected, when operated one by one? (to rule out any zigbee issues).
NB
Here I’ve found a screenshot of an previous app version, it definitely was there in any group type one day:
