XBee actual data rate lower than expected n dropping packets

I have the following setup:

Seeduino Stalker 328 + XBee XB24-B transmitting to XBee XB24-B + UArtsBee + USB

I am transmitting continuously at 9600 bps using the default configurations for both my XBees XB24-B.
Here is the actual data rate for a given program:

Wired connection (no XBee) : 7694 bps
from the ZNET 2.5 Router/End Device AT to ZNET 2.5 Coordinator AT: 6800 bps (some lost packets)
from the ZNET 2.5 Coordinator AT to ZNET 2.5 Router/End Device AT: 0356 bps (many lost packets)
from the Zigbee Router/End Device AT to Zigbee Coordinator AT : 0000 bps (doesn’t work)
from the Zigbee Coordinator AT to Zigbee Router/End Device AT : 0328 bps (many lost packets)

For higher baud rates (115200) I get even worse packet drop rates:

Wired connection (no XBee) : 94200 bps
using XBee XB24-B ZNet 2.5 : 27900 bps

Why is that? Is there anything I can do to improve these rates?