That’s the indisputable benchmark in this discussion. You are able to use the app to demonstrate a group of these lights can be turned on, in what is observed to be, concurrently.
Assuming this lighting protocol has no native group feature, the sheer speed of Wi-Fi masks the fact multiple commands are sent in sequence.
Nevertheless, something is clearly different in how the task is performed by the app vs Home Assistant. Wild guess: maybe the app doesn’t wait for acknowledgement from each light, before transmitting the command to the next light, whereas Home Assistant does. However, this would not explain why both the app and Home Assistant appeared to work equally well in a previous version, unless something recently changed in Home Assistant. 
+1
Comparing the two side-by-side would add another data point. If they were to behave identically, you would know something changed in the network. If they behave as you remember it (previous version handled groups better) then we have more proof the problem is software related.