I was hoping @arne_f would know. His tests in that same thread I linked to match my results and at one point he said the bottleneck is Cake
Regardless, my first post in this thread says to test for this when setting up your maximal bandwidth, and that lots of trial and error will be involved in fine-tuning QoS.
Arne’s post here suggests otherwise.
To clarify, my opening post in this topic is not asking for any solutions. It is simply to share my experiences with fine-tuning QoS in IPFire so that others may benefit.