I am running OpenWrt 22.03.2 on a Linksys WRT3200ACM.
With SQM QOS disabled, I see about 720 Mbps download speed as measured with Ookla Speedtest. With piece_of_cake.qos enabled, and download speed set to 684000 (95% of 720 Mbps), Speedtest only measures about 320 Mbps. Is that expected? I am the only one using network resources.
On my turris omnia, the best I could get it to shape reliably was 550/550 Mbps, for bi directionally saturating traffic with MTU 1500, unidirectional traffic might reach a bit higher but it depends on what else runs on the router...