@AB2000 

I have the same issue and opened a case with Tech Support.  My setup is fine in "Router Mode" but when I changed it to "AP Mode" my download speed reduced to half of what it should be.  The tech support engineer told me that this is how it is designed and I just can't imagine that to be the case since AP Mode seems to be pass-thru and should take less processing power.  Then I tested it out on the Deco X60 and I get over 900Mbps download and upload, so it' clearly not by design that you have slower performance in AP mode.  The the tech support engineer wrote this back:

Dear customer, 

Many thanks for your valued reply.  The Speed test result you have send is actually a good speed already.  Are you still experiencing the problem?  Thanks for your cooperation and patience.

We look forward to hearing back from you. If you have additional information or feedback to provide, please let us know.
Uriah Manuel Legaspi
TP-Link Technical Support

This is pretty incompetent technical support when they tell you that you get half for what you pay for, but that's okay because that's actually good speed already, meaning our company is not interested in fixing your problem.  I've asked the tech support to escalate the case to engineering, but he's gone dark on me.  TP-Link is pretty good product but I would have to say support is lacking.

@Tony if you work for TP-Link, how about taking a look at this and report it up to engineering.

@collabguy thanks for this. And you're right, that's a terrible answer. 

@AB2000 TP-Link needs to fix this for the M9+ system.  I suspect a lot of people don't have 1Gbps speed and not running into this issue, so there's not a broader complain about this.  I tested this out on the Deco X20 APs, and download and upload speed was expected with 900Mbps+ download and upload speed.  I'm hoping it's a firmware fix on M9+ and not a hardware limitation.  But we'll never know if TP-Link doesn't look into this.

@collabguy 

Support was asked to follow up with you, and they were informed of your experience so far.

With respect to the Deco M9 I've seen speed issues result from the devices being hot. When the speed drop and the M9 is inspected is it hotter than it normally is?

If router mode and ap mode are working in half look at the QoS settings to make sure those speeds are not set lower than 1000 Mbps. 

Within the Deco app, disable the 2.4G network, this forces your device to go to 5G (More > Wireless Settings)

Also, I am not sure if tried, but power down the other Deco nodes, and only have one M9 to the router. Wire a computer to the second node and test those speeds with Wi-Fi disabled on the computer.

Not sure if a Google Next router is being used, but that will place speed limitation on devices unless given priority which lasts for 4 hours.

@Tony Thank you Tony.  Interestingly, the download speed is fine in router mode but not AP mode.  Since the issue is in AP mode, QOS wouldn't be a factor because QOS is not enabled in AP mode.  Also, I'm only using two APs of three that came with the set, and tested both direct connect to my FIOS router as well as indirectly connected through the primary AP.  Neither changed the result in AP mode which exhibited the half speed.  I then removed both APs and tested on the third AP directly connected to the FIOS router, which should not be hot, and still have the half speed.  This only happens in AP mode.  When in Router mode, the speed is just fine.  I also tested out the X60 in different configurations and did not run into this speed issue.  I'm wondering if your engineering team can test this out in the lab.  It's reproducible 100% of the time in my setup.  Initially I though my FIOS router might be throttling the speed but I don't see any configuration in my FIOS router that would contribute to that.  Plus, the speed issue didn't occur in the X60 APs.