My first guess for the slightly higher CPU floor of the Rust version is that the Rust code has to do slightly more work per request, since it will free memory as it gets dropped, whereas the Go code doesn't do any freeing per request, but then gets hit with the periodic spike every two minutes where the entire heap has to be traversed for GC.
tokio 0.1 was definitely less efficient, when we compare go to 0.2, tokio uses less cpu consistently, even when compared to a cluster of the same size almost a year later with our growth over the time since we switched over.
