Auto upgrading of ST(wake up duration) when set it for less duration with longer SP(sleep period)

Working on Mesh topology XBee3 DigiMesh 2.4 RF module

We are facing a issue with automatically upgrading of wake up duration even though if we try to configure ST(Wake up duration)for 250 ms and with SP(Cyclic Sleep Period) more than 5 minutes.

What we try to do?
Initially we set SP to 15 min and observed ST (wake up duration) was 1.1 sec but we want to minimize the battery consumption. So we checked and came up with the Sleep Immediate(SI) command but still it is taking 590 ms wake up duration.

How we can reduce the wake up time further because we are sure it doesn’t take so much time for it to transmit few bytes of data with mesh networking managements. Since we are only sending 20-30 bytes of data and it is using 250Kbit for a second. We are not able to understand why it is taking so long.
Please advice us how to reduce wake up duration further. Is there any documentation available to calculate the wake up duration?