Oh, at most a second, but really less than a second. It results in a slight lag when used in a lighting automation. I could do a test to compare them though!
However, Iris V1 Motion Sensors don't have a light / lux sensor. It's important to understand that even with the Aqara sensor, the lux reporting capability is limited, because it's only reported just after motion detection is reported (and is restrained by the sensor's 60 second "cool down" time on motion detection). You mentioned you have "Xiaomi motion sensors" and if those are really the Xiaomi model then they don't have any light / lux sensor at all.
Well, because the Hubitat and SmartThings hubs have different ZigBee radios, the Zigbee mesh network routing topography is likely different, which means that end devices would likely "choose" a different parent (either the hub itself or a repeater) when you moved over to Hubitat. That could then reveal that some of your ZigBee repeater devices aren't "friendly" with your Xiaomi / Aqara devices.
I only have one myself, and it is the device that most frequently drops its connection, though it has remained connected for up to 6 months without interruption. Just based on everything I've read, the leak sensor has the most connection issues. I should also point out I have the "original" revision of the Aqara Leak sensor with the contacts that are screw heads. The "new" revision has button-face contacts that can't be unscrewed.
Perhaps the connection issues are due to interference from the metal parts? I'd love to try cracking mine open and add more of an antenna lead, but that would of course ruin it's use as a leak sensor because it needs to be waterproof!
My foggy crystal ball says most of us won't be able to get any of the new ZigBee 3.0 certified Aqara devices in our hands until the end of the year, or even early next year.
Yes the T1 devices are the ZigBee 3.0 certified ones. There are also some I've seen with "S2" added to the name which will be ZigBee 3.0.