I'm still confused. I digged a little deeper and found some reports of other MT7621 users getting double of that, some even here on the forums (like on this thread). The OP claimed ~200 to 300 Mbps with cake and over 400 Mbps with simplest_tbf.qos.
Another strange thing is, given simplest_tbf.qos is supposed to be lighter, it should achieve at least different results than piece_of_cake.qos, and yet I'm seeing the same ~150 Mbps throughput no matter what combination of qdisc and script I pick.
Given the inconsistent reports, I think I should investigate this further. With that in mind, what's the best way to monitor SQM QoS? I mean, how can I see the CPU usage and other technical info of an active SQM instance?