I get exactly the same problem here, I run a server & client.
All is going good, after have play some secs or mins it start to freeze randomly with the long ticks message.
I can wait the freeze and continue to play, sometimes it seen to go ok and I can play a longtime.
When I run only one instance of my project I get 120 fps or over.
When I run two instance with the multiplayer it running at around 60 70 fps.
I have try many configuration about the PIE mode but nothing really change.
I don’t have get the chance to test my project over the internet or on lan with multiple computers.
I have only test my project in the same computer.
I use unreal engine 4.18.1 with a nvidia 1060 gtx
You can see here in this video at 1:34secs and at 4:21secs and at 10:23 secs…
Some time the freeze is more long it never exactly same.