Just installed OPNsense, having some performance problems

[TuxPenPen]

Hi everyone, I've just installed OPNsense for the first time (and generally the first time setting up a router and firewall, so please excuse me if I write really dumb stuff).
I'm forced to use my ISP router (a Fastweb Nexxt, I'm in Italy), so I've set DMZ on the IP that I've assigned to my OPNsense: everything seems to work well (dowload speed got a bit worse from 800+ to 600, and upload got better from 700 to 900+, but this minor performance changes aren't really important to me), but sometimes, particularly when starting downloads of big files and torrents, the connection gets really slow or even stops completely and then restarts after some time. When this happens, on the opnsense interface it doesn't show the IP address of the IPv4 gateway, but only "...".
This initially made me think it was an ISP router problem, but I tried connecting a pc directly to it and everything works fine. What did I do wrong?

[cookiemonster]

Likely the connection between the two routers falters during heavy load. It can usually be unstable interfaces like usb-nics or realtek nics.
What's your hardware and any clues in dmesg in OPN sense? In case is new to you, connect to a console (ssh to OPN or from a screen/keyboard attached directly) and run that command. We're looking for clues there.
ps. SSH'in makes it easier because you can scroll through the buffer.