Hi,

Kamailio has some configuration options that logs a msg to the syslog when some config script action takes more time that is expected, the  options are:

• latency_limit_action=500
  
• https://www.kamailio.org/wiki/cookbooks/4.1.x/core#latency_limit_action
• latency_limit_db=500
  
• https://www.kamailio.org/wiki/cookbooks/4.1.x/core#latency_limit_db
• latency_log=2
  
• https://www.kamailio.org/wiki/cookbooks/4.1.x/core#latency_log 

The issue could also be related with the TCP write queue, maybe you have set low values on the following parameters:

• tcp_conn_wq_max 
• https://www.kamailio.org/wiki/cookbooks/4.1.x/core#tcp_conn_wq_max
• tcp_wq_max 
• https://www.kamailio.org/wiki/cookbooks/4.1.x/core#tcp_wq_max

Other hint is to look for the OS  send Queue, the package could be delayed because the other end is not reading it fast enough:

• netstat -altp
  

it also will be important try to isolate the issue in terms of SIP, the transaction delayed(UPDATE) is always the same in all situation that the issue occurs? 

Hope that the information above helped you.

Regards

José