Here is the TL;DR from my opening post for tweaking QoS in IPFire:
At the top of the QoS page, set the download and upload bandwidths to about 150% higher than the ISP’s rated bandwidth. So for a 100/100 connection, set these to 150/150.
For the upload/download Web classes, start at default speeds maximum and test on speedtest.net. You should notice speeds are equal to ISPs max speeds and the download and upload pings are higher than idle ping.
Start dropping the maximum upload/download speeds in small increments, say 10Mbps and retest on speedtest.net each time. When you see the download and upload pings start to drop to about equal to idle ping, then switch over to Waveform’s BufferBloat test, which takes longer but is more accurate. Now start dropping maximum upload/download web speeds in smaller increments until Waveform reports download and upload ping times of +0ms. That is your sweet spot. Now you can go to the rest of the upload and download classes and adjust the maximum speeds to the same as web or lower.
Lastly, adjust the guaranteed bandwidths so that the sum of all guaranteed bandwidths is the maximum speed you determined from testing.
Here is a table illustrating two examples. One is cable internet with a 250/25 speed limit. The other is fiber with 1 gig symmetric.
