@cegeto 

We will get some details to do further analysis and try to fix it. 

What is the current network topology/layout? And may I know who is your internet service provider?

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

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

With your computer or cellphone connected to the slave Deco only, please run the speed test for 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.

Please connect your smart phone to the Deco's wiif and login to the deco app; click the settings button and then choose advanced>>IPv4>>Internet connection type and uncheck obtain DNS automatically and change the primary DNS to 8.8.8.8 and click save.

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

Have a great day.