Собственно из статьи выше:
Flow is designed to type check extremely large projects. Over the past several years, we’ve introduced some big changes to enable Flow to scale exponentially to 10M+ lines of code, just barely keeping pace with Facebook’s codebases. For example, lazy mode only checks files affected by local changes, instead of the entire codebase. However, Flow still has to run type analysis on the dependencies of the files that changed, a fundamental inefficiency which multiplies the cost of each change.