It's wild to me that they would write performance focused code (unsafe ptrs, memory mapped files, etc) and then still use linq in the main work loop [0]. This implementation could go much much faster. Of course rewriting the parallelism and aggregation from linq is a decent amount of effort, but I'm betting that applying the lazy man's hack [1] would have decent gains.
[0] https://github.com/buybackoff/1brc/blob/main/1brc/App.cs#L12...