Download too slow, upload normal

[rhartmannh]

We are on our way to create our first OPNsense machine, which seems easy so far, but we hit an issue that download speeds are far below the lines's capabilities, while the uploads speed is as expected.

Two lines, one Unitymedia 280/25 MBit, one Telekom 50/10 MBit. They are configured as MulitWAN Failover, with the faster line as Tier 1.

In the beginning we had download speeds of 300 kB/s. Now we have changed network cards and disabled various interface hardware settings, resulting in 3 MB/s. Which is still only about 1/4 of what we reach via direct access to Unitymedia.

The only place where we see something out of order are packet loss rates on both interfaces that go to around 10% for a few seconds every few minutes.

Of course we checked cables, switches and network cards. Direct access using the Unitymedia network or Telekom network deliver the expected download rates - so the problem seems to be within the OPNsense machine. What is strange: the upload speed is not affected negatively.

The machine is a 4 core i5 650 with 3.2 GHz and 12 GB memory, and the CPU mostly seems to be bored.
OPNSense 18.1.6