Not for a segment that doesn't exist. When I create a segment, it goes back over all nearby activities since the dawn of Strava time. That's the part where geographic sharding helps. Then, it has to check each one to see whether it traversed the segment without interruption or deviation - not within one meter but AFAICT within only a few. Does further sharding/tiling help there? Yes, and I'm sure that's part of how it works, but I'm interested in exactly how they apply those techniques and then solve all of the remaining problems - undoubtedly including those that neither you nor I have thought of. As you yourself say, there are probably some clever choices involved, and that's the part I'm curious about. It definitely seems like a "meaty" enough topic to fill out a blog post or two, and the world needs more such IMO.