RBR50 Download Speeds Diminish Over Time

This happened to me on my RBR20. The 40Mbit bottom speed was the same as mine (my full speed is 200Mbit) which stayed until a reboot. It might take a week for the slowdown to start, but once it does, I was stuck at 40-45Mbit until a reboot.

Here was the cause in my situation - long story, but -

I have a NETGEAR (ironically) Wi-Fi dongle that is connected to my satellite TV receiver. The dongle allows downloading the program schedule. In configuring the dongle, it showed TWO duplicate network names for 5Ghz, and TWO for 2.4GHz. I suspect that the connection was to the backhaul and not the proper network. Again, they both had the same network name. This is done thru the satellite receiver, which probably gets confused when it sees a mesh network. This is a >10 year old, old-school DISH Network receiver.

I changed to the other 5GHz network (again, duplicate name) and since then the problem has disappeared. I consider this a bug in the router.

So, for your case, I'd go thru all your Wi-Fi connected devices and try to see which one may be causing the slowdown.