API Mode transmit packet time

i use XBEE Series 2 modules with UART speed 115200.
when i use ZNET 2.5 (API ) firmware for coordinator (1147) and for end device (1347)
API transmit request for 4 packets of 70 bytes ( summ is 280 bytes ) executes for about 130-140 msec ( i measure time between send first transmit request and recive last transmit status)

but when i upload XB24-ZB firmware to coordinator (2141) and end device (2941)
transfer of 280 bytes is about 180 msec from end device to coordinator
and 500 msec from coordinator to enddevice.

thank you