Short update:

I tested a Ruckus AP and an EAP670 in standalone mode after factory reseting both:

As you see, the Ruckus AP has full speed and the EAP on 5Ghz has a maximum of 9Mbit/s...
Upload is no problem. This is the provider speed.. Provider Download should be about 350Mbit/s.
 

  @Hank21 

Hello, thanks for your message.

So i spent the whole day with testing different scenarios.

I used following cables and lengths for testing.

Cat5,Cat5e,Cat6,Cat6A,Cat7 - UTP / S-FTP - each with 0.5m, 1m and 2 meters long.

I tested with six different EAP670s, one EAP610 and one non TP-Link product, a Ruckus T300 AP.

I also put away all other TP-Link products, Routers, switches and so on.

So my test setup was:

Internet -> FTTH Provider Modem -> AP

With the Ruckus i get instant full speed on download and upload.

All EAPS have full UPload Speed, but only downloading with about 2Mbit/s.

Further tests made on internal LAN. I added a Linux machine with an iperf3 server.

Instantly i got full internal LAN Speed with up and download. (All EAPS and Ruckus work perfectly)

So, the problem is located when the EAPs get out of LAN to Internet. Ruckus and wired clients working on full provider internet speed when they get out of LAN to Internet.

What else can i test or change settings on the EAPs?

Thanks for your time.

  @Hank21 

I found a solution. I downgraded the firmware of all six EAP670 Access Points to version 1.0.6

Works now.

Thanks.

@patchman 

how did you downgrade? Every time I try, I get an error that it cannot do it. 

I am about ready to send these things back

I'm having the same issues, 5ghz download speeds are about a third or half of the upload speed.

One of my APs (bought before the other one) supports downgrading to firmware 1.0.6, the other one came with 1.0.9, which won't let me downgrade to 1.0.6.

I've tried the 1.0.11 and 1.0.12 firmware and it still showed the same issue.
 

Speed test from my phone here:

Speed test from my desktop with a 2.5G NIC (same LAN the AP is plugged into):

I'm experiencing exactly the same issues on my EAP670 with fw 1.0.12. After downgrading to 1.0.6 everything is working normally.