Actually, the API isn't really any faster or slower.
If you find it faster to query NuGet.org's "v3" API endpoint, the reason is that it's a static index hosted on a massive Azure server farm. Your ProGet server is considerably less powerful. In addition, ProGet has connectors, authentication, package filtering, etc., and it would never be feasible or useful to generate a static index like NuGet.org does.
However, it is on our 2019 roadmap to implement this API, possibly as a new feed type or as an alternate API endpoint for an existing feed. But even so, if you need performance, you may still need likely need to stand up several ProGet servers in a load-balanced manner.