@Sandro_Becker 

Hi, Have you checked the Ethernet speed from the main Deco?

For Deco M5, its wireless link speed is up to 867 Mbps over 5GHz, and the expected wireless download speed is about 300-450m/s. So your current wireless speed is not bad. Considering the overall internet bandwidth is about 600m/s, you could consider our Wi-Fi 6 Deco Mesh, like Deco X20 or Deco X50:

https://www.tp-link.com/en/home-networking/deco/?filterby=6271%7C6093%7C5730

Thank you very much.

Best regards.