If you haven't tried this already, try increasing the size of the packet payload to 100,000 bytes and increase the transmission timeout (maybe 20,000 ms). The throughput test is measuring only the transfer rate of the payload/data portion of the packets. However, each packet contains several bytes of header information. So, if you send the same amount of information in fewer packets you will increase the transfer ratio.
If your module uses the 802.15.4 protocol, the maximum throughput you can expect is 83 kbps, according to the manual (page 11).
If your module uses ZigBee, the max is 58 kbps, according to the manual (page 84)
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