Usually its a precomputed list at run time. Think of a tree that goes from network address to locale(s). More specific network prefixes (probably) have more specific locales.

Companies like MaxMind collect and repackage this based off of information like RIRs, SWIP, DNS PTRs, or even end user billing/shipping addresses in online transactions. I dont think latency is used too much these days. Check out https://www.maxmind.com/en/geoip2-databases for some examples. I'd say that level of detail is about industry standard and most companies would simply pay maxmind or another vendor for this data.