Indeed, we are rendering the whole page with the javascript, that's why it takes longer than a curl. For now, it's especially useful for dynamic pages but we also plan on supporting pages that don't require rendering.
Maybe you already do it, but I think integrating adblocker functionality when loading JS sites would be desirable to reduce load time. And if ads are what the API user is interested in, perhaps add a flag for whether or not one wants ads to load.
Recommendation: https://github.com/cliqz-oss/adblocker Should be the fastest adblocker library (used by Ghostery, Cliqz and Brave)