Do you have a iperf T/P test before going to publishing messages using MQTT ?
or running your application in our 43907 evb to have a compare also.
and kindly enable some logs for debug...
Have you defined TX and RX packet pool size on your application makefile?
If it is not defined then, by default the RX and TX pool size will be 7.
Less number of pool size can be the reason for lower throughput.@
If not, have a look on the following guide which may help you:
Would you please share current status ? thanks.