See pictures posted here:
I do see the target SOC changing every 7,5 minutes and not 15 minutes.
Sometimes the target SOC in between a 15 minute block is 1% off and this stops charging or discharging.
I don’t understand why you would change SOC in between a 15 minute block, but it happens.
Recalculating of target SOC is only needed if real SOC is off from Target SOC by 0,5% (or user adjustable) at end of a 15 minute time block.
This would real lower your server SOC recalculation time.