RBR850 WiFi speed

What device is measuring the WiFi Link Rate on  these two routers?

just my laptop, wifi information and speed tests

According to the Product Data Sheet, the maximum theoretical Link Rate on the 5G WiFi band is 2400Mbps

This would be with the WiFi device (literally) within inches of the router.  WiFi Link Rate falls off dramatically with distance and material between the router and device.  (p.s. I have no idea what Dim Bulb thought to list the aggregate speed (2400 + 2400 + 1200)  backward from the radio list (1, 2, 3).  It is glaringly obvious that the 2.4G radio is the one that has a theoretical maximum of 1200Mbps)

If the laptop is connecting to both WiFi routers at 5G and the same relative distance, I have no explanation why one would be twice the Link Rate of the other.

Note also that Link Rate is not the same as Internet Speed.  If the Orbi WAN port (2.5G) is connected to the Zyxel 2.5G LAN port, then measured internet speed should be similar when connected to either router.

Thanks for the replies, I have just found a page that says the RBR850 doesnt support 160MHzcould that be the issue?

I believe this is correct that the RBR850 supports 80MHz wide channels and does not support 160MH channels.*

Not clear (to me) what the Zyxel supports.

Of course, the test device would also have to support 160MHz channels.

*The RBR850 is based on the Qualcomm IPQ8074A WiFi chip.

Although the Qualcomm web site appears to be messed up (no specs), this site has the following:

https://techship.com/product/compex-hk01-embedded-board/?variant=003

It might also depend on how many antennas the test device supports.  (MIMO)