problem may not come from this connection but another one (think about FTP control vs data connection) , you may experience slow data connection start because control session is acting weird or distrutped. As an example, may be the client is trying to resolve server name from dns, but dns is blocked on primary server so you take 3 seconds delay until secondary DNS server is queried and answers Do a PCAP to prove that firewall delays or not the packets of the connections that devs say they have a problem with. If PAN delays packet, open a TAC case. if PAN doesnt delay packets, tell devs to provide better troubleshooting because whatever the problem is (PANOS or other) they are not pointing which flow is not working.
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