RBR50 - Static routes very slow transfers

No problem! Cascaded means that you basically have one router plugged into another and each is managing their own LANs. There's an ethernet cable that runs from a LAN port on the Orbi router to the WAN port on a second router. The Orbi assigns an IP to the second router, serves internet to it, etc. By default, computers/devices on the Orbi LAN can't see anything inside the second LAN. By using the WAN port on the second router for the connection, that router is basically assuming that any traffic happening on the other side of that port is "outside" traffic and ignores it unless you have firewall or passthrough rules in place.

In my case, the Orbi LAN lives at 192.168.0.0 and the secondary LAN lives at 10.0.1.0. The static route tells any computers on 192.168.0.0 that have traffic destined for 10.0.1.0 to direct it to whatever the ip address is of the second router. That router catches that traffic, and if it's allowed through the firewall then it sends it to the appropriate destination on its own LAN.

With the issue I'm seeing, I don't recall it being so bad about a month or two ago when I set it up. There have been a couple of firmware updates in the meantime and I'm wondering if something came off the rails in a recent firmware. The way the transfer speeds of files "into" the second lan fluctuates is very odd and it's suspect that it's only happening in one direction and dependent on which side of the router the request is being made.