Thanks for your reply.
If I well understand the link :
When we using 802.15.4 (Xbee PRo) :
@115200 bauds : Tlat = 8 ms ; Ttot = 15.9 ms.
@9600 bauds : Talt = 41.3 ms ; Ttot = 82.9 ms
As I'm using 115200 bauds to communicate with xbee module I should have a maximum value of 15.9 ms right ?
Thanks for your feedback.