The 2.5 Gig port of Orbi is limiting network speeds received by the ISP Modem

I ordered the Netgear MS305 2.5g switch (which is currently on sale) to replace my TRENDNet TEG-S350 2.5g switch in my garage as it has a known issue causing ghost satellites to show up in the orbi app (no other noticeable issues, but figured this problem was annoying enough to solve because it also causes the firmware update in the app to error out).  The new switch resolved the issue with the ghost satellites and so far is performing very well.  

I also tested putting the TRENDNet 2.5g switch in front of the RBR850 connected to the 2.5mGig port of the XB8.  This is where things get interesting.  The WiFi speed I got directly from the RBR850 was insanely fast (840Mbps).  Additionally, the LAN speeds from my ethernet backhaul SAT were only reduced to 700-750Mbps (instead of 450Mbps w/out the switch between the XB8 over the 2.5g port).  LAN Internet speeds directly to the RBR were around 800Mbps, BUT  speeds using a 1G usb-c adapter directly on the 2.5g switch was also reduced to around 800Mbps (getting the full 940Mbps when connected directly to any XB8 1GbE port).  Connecting a 2.5g usb-c adapter to the switch of course provided full internet speed (1150Mbps).  My conclusion is that although the 2.5g switch helps the Orbi, it actually has similar issues with the XB8 when connecting 1GbE devices to it (slightly reduced speeds vs using a 1GbE port on the XB8).  This leads me to believe the issue is with the Xfinity XB8 modem and not the Orbi.

How was the speed test on the RBR 850 Orbi itself?

The speed test on the Orbi app was 1150Mbps but speed from the RBR directly over a LAN port was around 830Mbps.  That same connection on the RBR850 consistently gets 940Mbps when using the 1GB port on the XB8 so using the switch is still a little slower with the XB8 and the 2.5g port.

I might add my modem is an XB7. Was going to upgrade to XB8 but looks like you are using the 8. Might try it anyway. 

gregnauman Yeah, I have the XB8 but since you already have the same issue with the XB7 it doesn't hurt to upgrade.  If the issue is hardware, you could get lucky and not have it.  If it's software (which I think it is), I'm sure it will eventually be fixed since it seems to impact more than just the Orbi.  The XB8 also has WiFi 6E which could be nice depending on how you plan to have things configured.

Musicman

Well I upgraded the modem to the X8. Got it home and setup, placed it in bridge mode. 

Same exact issue.

1.Plug orbi into the X8 2.5 gig port orbi app gets 1441 down and 250 up

Anything plugged into the Orbi ethernet ports goes initially to 900 and then tapers to 150. Upload is fine at 250

2.Plug orbi into the X8 1 gig port orbi app gets 964 down and 250 up

Anything plugged into the Orbi ethernet ports goes to 945 up and 250 down

3. Wifi speeds are a sustained 650-800 either way.

Makes you wonder what the problem is - my XB7 and 960 do not have this issue.  Seems more commonly reported with the XB8, but I wonder if later versions of the XB7 have it and whatever "that" is has carried over into the XB8.  Our XB7 is probably two years old.  If that's the case, it may be a hardware compatibility problem in that I'd assume our XB7s are running the same firmware.  Mine says:

I do hope it gets sorted because certainly at some point we'll probably get pushed toward an XB8 or whatever comes next.

bullm00n my firmware is the exact same as yours except it is CGM4981COM_6.7p14s1_PROD_sey.  

gregnauman wifi speeds on the RBR850 weren't impacted when using the 2.5GB port on the XB8, but WiFi over satellites was a little slower when using the XB8 2.5mGig port likely due to the reduced LAN speeds (closer to 500Mbps instead of 700Mbps).  My satellites are connected with ethernet backhaul, so that may be why.  Since the same behavior happens on my 2.5GB switch I'm thinking this is xfinity's issue.  I also saw a similar thread posted on the eero reddit about reduced speeds with the XB8 despite the network being connected over ethernet.