I've used Postgis for read only data with indexes on the spatial data, maybe 25mm rows with multiple joins and on cheap hardware it can easily return results in

I've used Postgis for read only data with indexes on the spatial data, maybe 25mm rows with multiple joins and on cheap hardware it can easily return results in under 100ms. Nowhere near 500qps, though. Depending on your application, st_within vs st_intersects may have drastically different performance, too.