Incoming messages take too long to show up

Hi everyone. I'm having problems with incoming messages during high 
traffic. During a campaign the other day I was receving 85 messages per 
second, however, clients were complaining that my reply took over five 
minutes. I checked the logs, and I found out that wasn't the case, I was 
replying only a few seconds after Kannel was logging the message. 
However, my Carrier sent me a tcpdump were they showed that Kannel was 
in fact replying to the incoming message with a Deliver_sm "OK" with 
only a few seconds of delay, but that was minutes before the message was 
logged in Kannel logs. Where does the message go during that time? What 
can I improve?