One problem is that a change in a.js may trigger a new error in b.js.

ESLint could also cache things fairly trivially:

  hash = hash_file_contents()
  if previously_seen_hashes.contains(hash)
      report_previous_results()
  else
      run_lint_and_cache_results()
  end

maybe that already exists. But that has the same problem.

When you've got enough hardware to throw at it, then "just run it on the full code" is the safest.