Network issues with USGflex-50

Hi,

I fell into the Zyxel trap I think. A client of mine bought a Zyxel USGflex-50 because the old PF Sense firewall broke down.

The main router has two vlans, one for guests and the other one, protected by the Zyxel for the internal network.

When we installed it, it made the whole network slow as hell. Not only did some applications that need internet stop working, but the internal network is slow as hell as well.

I switched everything off, tried a million things and still it is slow and the applications can't connect to the internet. If we use the browser, there is no problem, every webpage is accessible.

Does anyone have any idea what it could be?