Server has extremely now avg tick

Hey there,
lately I started my own minecraft server for me and my friends on my pc…
The problem is, that my average tick is constantly between 5 and 9 (for example in the moment: 5.445)
My TPS seems to be normal tho (TPS from last 1m, 5m, 15m: 20.0, 20.0, 20.0)
My frinds constantly talk about lag, when they connect to my server through my ip (I use noip for ddns) they got lag. but when I connect through localhost or 0 it works perfectly. (it lags too if I connect via ddns adress)

My specs:
CPU: AMD Ryzen 7 3800X
Allocated RAM: 4GB 3200MHz
(I’m also using Aikars flags)

I hope anyone can help me :smiley: