I hope are you doing great!
I have the following problem.
In my firewall I have 2 interfaces "inside" and "outside". When I try to reache the LAN behind outside via LAN behind inside, I have a high delay but when I send a ping request from my firewall for any one lan I haven't this issue.
The problem only happen when the packege pass the firewall.
I checked the traffic, connections, cpu process, memory and everything ok. I searched for a bug but I can't find something that help me.