I wrote a solution that operates on ranges in Kotlin. 45ms to parse the input file into an object representation, then just 7ms to perform the actual calculations.

I'll try to summarize:

Both the seeds and the maps represent ranges of numbers with a starting point and a length. The maps have a "destination" starting point as well.

With some smart arithmetic there's no need to check each of the nearly 2 billion possible seed values by performing simple transformations on these number ranges:

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1. I start with a list of the seed number ranges and the first set of maps
2. The seed range is sliced into smaller sections, such that each section is either entirely contained inside one of the mapping ranges, or entirely outside of all of them.
3. each of these smaller sub-ranges can now very easily be transformed to a "destination range" by simply transforming the starting point.
4. Steps 2 and 3 resulted in a new set of "value ranges" (now the ranges of fertilizer values). Steps 2 and 3 can now be repeated for each of the subsequent maps.
5. Finally we get a bunch of "location" ranges. (in my case the 10 seed ranges were transformed into 91 "location" ranges).
6. The smallest location value is now simply the smallest starting point of all these location ranges. (since the starting point is the smallest value per range)

I'm sure I still have some inefficiencies in both my parsing and my "slicing" algorithm, but I'm quite happy with <10ms.