The openwrt docs reference this post: forum.openwrt.org/t/so-you-have-500mbps-1gbps-fiber-and-need-a-router-read-this-first/90305

They talk about a router from 2009 not being able to handle CPU-only SQM on a gigabit internet connection.

If you have a gigabit internet connection you don't need SQM.