@ribrob 

The hardware version is printed on the back panel of the actual device. Below is the instruction: https://www.tp-link.com/support/faq/46/

As for the speed issue, we need do more troubleshooting to locate this issue and try to figure it out. 

1. What is the current network topology/layout? And may I know who is your internet service provider? And what's the bandwidth you pay for them?

2. With your device connected to the main Deco by wired cable or wireless, what is the actual speed you can get separately? And please click here to check the wired/wireless negotiation speed.
Note: The steps are the same, while you need right click on local area connection or ethernet connection to get the wired negotiation speed.      

3. Besides, please try to run a speed test via the Deco app and show us the results. 

4. With your computer or cellphone connected to the slave Deco only, please run the speed test 3 times and show us the test results; Furthermore, please connect your computer to the slave Deco by ethernet cable to verify the wired speed.

5. Try to change the DNS server to 8.8.8.8/8.8.4.4 under cadvanced>>IPv4>>Internet connection type page.

5. Besides, try to swap the locations of the slave Deco units or switch the slave Deco to the main one to give it a shot.

After that, please run the speed test again and check whether it work.

Have a great day.