I noticed recently slow internet speeds, and I'm pretty sure the problem lies with the Deco M4. I have a 1 Gbps connection from my ISP, but average speed with SpeedTest.net is around 250 Mbps. I tested via WiFi on my phone and hardwired to my PC (Modem -> Main Deco -> PC).
Modem speedtest is around 900 Mbps and it is also the speed I get when I connect my PC directly to my modem.
The problem seems somewhat recent as I didn't have this problem in the past. I'm also running a pihole as my DNS server, and still have this problem if I switch to Google or CloudFlare DNS.
I rebooted the main Deco as well as unplugged it with no change.
Thanks for your help