Git’s Database Internals III: File History Queries (GitHub blog)

If these history modes usually have the same output, then why wouldn’t we always use --full-history --simplify-merges? The reason is performance. Not only does simplified history speed up the query by skipping a large portion of commits, it also allows iterative output. The simplified history can output portions of the history without walking the entire history. By contrast, the --simplify-merges algorithm is defined recursively starting at commits with no parents. Git cannot output a single result until walking all reachable commits and computing their diffs on the input path. This can be extremely slow for large repositories.