From the Qualcomm website on the IPQ4019
Wi-Fi Peak Speed: Up to 1.733 Gbps
What is peak speed?
Let's start with what it is not:
It is not wan to lan with NAT and firewalling. It is not more than 10' in distance. It is not one 1GB port to any or all others.
What is it? it a perfect situation in the local network that 4 717MHz CPUs can handle until the heat throttles them down.
The radio capabilities are not a factor for your question: "Why does SQM bring my 4 717MHz CPUs to a crawl?"
This explains why.
But, specifically, this does the numbers for you.
Evidently in an ideal world, we should have maybe 1.2GHz processors or better, and maybe have two cores at least one can handle interrupts on the receive interface, and one can handle interrupts on the send interface, and they can share the firewall and queueing duties.
It takes 2 of your 717MHz cores just to keep up with one set of those jobs.
Read the rest because I'm not going answer anything that @dlakelan took the time to cover.
@JonP has the best advice for you, which is
It does work well on my Pi4, so it is not just a report anymore: it works on the Pi4.
We are not to blame for your throughput. Challenging our explanations is not going to change physics.
Upgrade to the latest stable firmware, which is hopefully 23.05.2 for you and enable packet steering across all 4 cores.