Did a quick comparison siphon draining cut-off time with higher in-flow.
Below all the pump output into this single growbed at around 1600 ~ 1800 Liter per hour inflow as specification on pump taking into consideration of water head of around 0.8 meter.
It too a whooping 12 minutes and 30 seconds to total cut out as compared to installation below which has half the above in-flow and takes 1 minute 22 seconds.
Its about 11 minutes 8 seconds for siphon to cut out considering the outflow is the same for same size siphon and no changes. This time can be reduce if we adjust the siphon outlet as in photo below.
If we increase the first vertical out and have shorter horizontal without the second elbow part downwards this installation can handle more inflow at a better cut-off time.
I will try again with a test rig, experimenting on various outlet to cut the siphon cut-off time to make it less than a minute or best less than 30 seconds after water has drained out.
But on this setup its for around 500 ~ 1000 liter per hour in-flow that will give less than 2 minutes total draining time for this size of growbed.