Enabling SQM causes bufferfloat to go from B to C on a cable connection

You reloaded the driver/rebooted your laptop after the change, right ? I used to have 6205 earlier, and I was able to get slightly better speeds than the 55 Mbps that you are seeing, but it wasn't too much better. I think 100 is your absolute max.

My bad! I thought it was related to the router in question:-). I got 85 mbit/s now so it really does make a difference, thanks! Now I only need the router itself to get fixed:-).

Blogic recently uploaded a commit to his staging tree which could solve the problem with mt7621 devices like the DIR-860L B1. If you want to test a build which incorporates the commit and SQM QoS try my updated build here.

Since this thread diverged from its original topic quite a lot, I will just note for posterity my latest struggles with wifi speed: turns out it can have quite a huge impact if you have a 20cm thick concrete wall in your apartment! Also, if your router has external antennas, it pays to fiddle with them and try which positioning works best (I think they should point to various dimensions optionally, i.e. into X, Y, and Z axis of three dimensional space). I am now using TP-Link Archer C7 v2 and facing the router, I was able achieving 150 mbits over N (router was set to AC and 80 Mhz band, but I gather the wifi card in my laptop is only able to use N and 40 Mhz). When the laptop is on my desk, I was able to get around 100mbits with correctly positioning the antennas up from 60mbits when they were all pointing into one direction. Moving the router by one meter (so there are no plasterboard walls in the way, just the concrete) improved the speed by about 20 mbits. Wifi is really sensitive to positioning, and this is all in a 86 m2 appartment, not a big house!

Though, back to original topic, since the speed of my connection varies with the position of my laptop in the appartment, so should probably vary the SQM limits. Now I have it set up to 142 mbit for the speed of my upstream link, which is 150 mbit. However, it means the wifi itself is not debufferflowed:-(. Now I get a B on DSLreports, which I guess is still decent, I guess.