You have more than enough cores for each then. Probably too many. As a test, try the qbit or jellyfin one in host mode and see if the network performance changes. I’d start going down the rabbit hole of tuning bridged mode network in LXC, or just keep them on host mode.
Bad RAM wouldn’t present like this. You’d more than likely never get past boot with a DDR5 board having caught it with POST tests, or you’d have thrown a kernel exception by now.
I saw you mentioned that a new LXC container didn’t have the traffic problem, so this is definitely something with config somehow.