I've a wireless card installed on a gentoo laptop which works fine on acx drivers. I've forced the wireless router to operate on 11Mbps so as the card. The problem I have is that using a udp socket and trying to send 1000 packets of 1500 bytes size(MTU and in the 1500 bytes I've included the IP header so no fragmentation on IP layer) there is a strange delay. Meaning that if the first packet is sent at time 0 the last one is sent 4 seconds later! This problem does not appear running the same code on my desktop computer but on an ethernet interface. What could be the cause of this delay?

It needs investigation and look into the code, but mu guess is that you fill system buffers and then need to wait until there's free space.

To see if it's true
1) Monitor the traffic you send. See what is the delay between the messages.
2) Show (print) the codes you get from the system calls. You may be getting EAGAIN or such from sendto().