Today I learned that “Smart Queues”, the Quality of Service integration for Unifi devices, on a “Unifi Dream Machine” can drastically impact your connection performance. To put this into perspective, that expected speed on this connection were 100Mbit/s down, 40 Mbit/s up. Before disabling the “Smart Queues”-feature any speed test would show up as 25Mbit down and ~3Mbit/s up. Afterwards, everything came out with the expected speeds.
I came across this while investigating the under delivery of internet speeds by my ISP. I started to do various speed tests at various hops throughout the network and it turns out, my ISP was delivering exactly as promised and the bottleneck was actually my “Unifi Dream Machine”. After disabling smart queues things worked as expected. I came across this hint on Reddit, which lead to the solution