Starting a net new post here - I see others have the same issue with no response from TP Link support. 

I have a 1Gbps up/down ISP, which at the carriers equipment I see true Gig speeds. This includes equipment directly connected to the carriers router.

I have 2 M9's approx. 25' apart (one in the basement and one on the second story) They are directly hardwired (CAT6) together. The primary M9 is connected directly to the carriers router, and as mentioned the second port is directly connected to the second M9. Wired into the second M9 is a PC with a 1Gbps NIC set to 1Gbps/full duplex. 

The primary M9 shows carrier speeds over 700Mbps when performing the internet speed test from the Deco app. Wirless devices connected to the primary M9 (iPhone/iPad/MacBook) all see throughput above 300Mbps. I would expect this given it's wifi. When connected to the secondary M9, speeds appear to be capped at 100Mbps, this is true for both wifi and ethernet connected devices. I have direct connected via ethernet into the primary M9 and see the same bandwidth, under 100Mbps. Connecting direct to the carrier equipment I see >800Mbps. (same PC, same cables)

At TP Link phone support request I have completed a hard reset on both M9's and setup from scratch. I have also, swapped the M9 locations and assignment (primary/secondary) with  a hard reset and reconfiguration. In any scenario devices hardwired to the primary M9 cannot exceed 100Mbps throughput. The only time I am able to exceed these speeds is with devices connected via wifi directly to the primary M9. 

I have run in both router mode and access point mode. I have run with everything disabled (anti-virus, QoS, parentla controls). No matter the configuration the same bandwidth challenges exist. I have also run file copy tests between devices on my network, anything going through the M9's is limited to under 100Mbps, with the M9's out of the picture or routed around, I see full 1Gbps copies across my LAN. 

If I adjust the total bandwidth setting under QoS to anything under 100Mbps, I see my devices capped at these speeds, expected behaviour. If I adjust this setting to anything between 100 and 1024 (the highest point accepted in the app) I see approx. 10% throughput, example if I set this to 500/500Mbps I see a bandwidth at 50/50Mbps, as I adjust this number higher, ie 800Mbps I see speeds capped at 80Mbps. At the maximum setting (to meet my providers bandwidth) 1024Mbps I appear to be capped at 100Mbps. This is only true for devices not connected to the primary unit via wifi, anything connected to the primary via CAT6 shows the throttled behaviour. 

I have also perfomed a speedtest from multiple devices simultaneously all connected to the same M9. The outcome, combined speedtest total do not exceed 100Mbps. The bandwidth is not equally shared, showing each device giving up some speed as the next device performs a speedtest. USing 3 devices the first maxed out at (upload speeds) 60Mbps, 2nd at 30Mbps and the 3rd at 10Mbps. 

I have to speculate that:  a) the firmware is capping the badnwidht/throughput per device at a maximum of 10% of the allotted bandwidth. b) the network adapter/settings are not running at 1Gbps, likely only 100Mbps. c) there is an algorithm issue in the bandwidth/QoS settings/configuration. d) a wired backhaul network is not used/fully used. e) TP Link Deco M9 Plus is not capable of delivering more than 100Mbps beyond the primary device.

I have seen other threads on this subject, and do not beleive I am the only one with this issue. I also don't beleive that one of the devices is bad, given I've swapped their positions and reconfigured from scratch with the same results. There is something bigger here that TP Link needs to address. 

I am not able to return for a refund (I have the receipt and box, but the store does not take equipment back after 30 days - I just found this issue in the last two days) TP Link 2nd line support doesn't return calls and the 1st line is unable to offer any further assistance. I'd like this to be addressed by TP Link support and feel I am owed support given I've spent nearly $300 (Canadian) on your products that do not deliver as promised.