Another wild guess: Lichess could be pre-calculating and caching the legal moves for the most common chess positions. While pre-calculating every possible legal move for every position would be impossible, you could pre-calculate the most common openings and endgames, which could cover a lot of real-world positions. This cache could easily be larger then practical for the client, but a server could hold onto it no problem. This could save on the net processing time, compared to the client determining all legal moves for every position.
Given that a good chess move generator will work in way less than a microsecond (TBH, probably even less than taking a DRAM lookup for a large hash table), and most chess positions have never been seen before, having a cache sounds counterproductive.
