Internet routing can often be sub-optimal, with the chosen routes providing worse performance than other available policy-compliant routes. This stems from the lack of visibility into route performance at the network layer. While this is an old problem, we argue that recent advances in programmable hardware finally open up the possibility of performance-aware routing in a deployable, BGP-compatible manner. We introduce RouteScout, a hybrid hardware/software system supporting performance-based routing at ISP scale. In the data plane, RouteScoutleverages P4-enabled hardware to monitor performance across policy-compliant route choices for each destination, at line-rate and with a small memory footprint. RouteScout'scontrol plane then asynchronously pulls aggregated performance metrics to synthesize a performance-aware forwarding policy. We show that RouteScoutcan monitor performance across most of an ISP's traffic, using only 4 MB of memory. Further, its control can flexibly satisfy a variety of operator objectives, with sub-second operating times.