Delay when sending GET or POST requests to API REST from PHP script

Hi,

 

I'm trying to send GET and POST requests to API REST using a php script. It works fine, except that F5 LTM needs between 15 to 20 seconds to reply. When I test exact same request with curl there is no significant delay to get the reply. It seems to be specific to php script. I also tryed to capture packet on F5. The F5 receive the query instantly (no significant network delay), then no packet send or received during 15 sec, then F5 reply.

 

I don't understand what the F5 does during this time. Do you have any idea ?