The Xbee shield "should" be designed correctly. It's only Faludi's book that's screwed up.
Please explain what you think the book says, and why you think it is screwed up.
How on earth can someone make such a basic error in a published book?
People make mistakes. It happens.
It's even possible, however unlikely, that you are the one that made the mistake.
In regards the delay, check the default parameter settings in the Xbee. It may be waiting
for a complete packet in the queue [eg, 100 bytes] before transmitting, etc.
Also, know that broadcast mode (specific send to address) is orders of magnitude slower than direct transmission.
You haven't defined how the XBees are configured, but, it sounds like you are sending messages in broadcast mode. That is going to be slow, because the XBees need to coordinate a quiet time for the broadcast to occur. Sending a direct message twice, to the tow different receivers is going to be orders of magnitude faster.