diff --git a/shadowmire.py b/shadowmire.py
index 2bb13d4..b9ddd47 100755
--- a/shadowmire.py
+++ b/shadowmire.py
@@ -36,6 +36,8 @@ USER_AGENT = "Shadowmire (https://github.com/taoky/shadowmire)"
 
 # Note that it's suggested to use only 3 workers for PyPI.
 WORKERS = int(os.environ.get("SHADOWMIRE_WORKERS", "3"))
+# Use threads to parallelize verification local IO
+IOWORKERS = int(os.environ.get("SHADOWMIRE_IOWORKERS", "5"))
 # A safety net -- to avoid upstream issues casuing too many packages removed when determinating sync plan.
 MAX_DELETION = int(os.environ.get("SHADOWMIRE_MAX_DELETION", "50000"))
 
@@ -516,13 +518,11 @@ class SyncBase:
         json_files: set[str],
         compare_size: bool,
     ) -> bool:
-        to_update = []
-        for package_name in tqdm(package_names, desc="Checking consistency"):
+        def is_consistent(package_name: str) -> bool:
             if package_name not in json_files:
                 # save a newfstatat() when name already in json_files
                 logger.info("add %s as it does not have json API file", package_name)
-                to_update.append(package_name)
-                continue
+                return False
             package_simple_path = self.simple_dir / package_name
             html_simple = package_simple_path / "index.html"
             htmlv1_simple = package_simple_path / "index.v1_html"
@@ -539,8 +539,7 @@ class SyncBase:
                     "add %s as it does not have index.v1_html or index.v1_json",
                     package_name,
                 )
-                to_update.append(package_name)
-                continue
+                return False
             if (
                 hrefs_html is None
                 or hrefsize_json is None
@@ -548,20 +547,17 @@ class SyncBase:
             ):
                 # something unexpected happens...
                 logger.info("add %s as its indexes are not consistent", package_name)
-                to_update.append(package_name)
-                continue
+                return False
 
             # OK, check if all hrefs have corresponding files
             if self.sync_packages:
-                should_update = False
                 for href, size in hrefsize_json:
                     dest = Path(normpath(package_simple_path / href))
                     try:
                         dest_stat = dest.stat()
                     except FileNotFoundError:
                         logger.info("add %s as it's missing packages", package_name)
-                        should_update = True
-                        break
+                        return False
                     if compare_size and size != -1:
                         dest_size = dest_stat.st_size
                         if dest_size != size:
@@ -571,10 +567,37 @@ class SyncBase:
                                 dest_size,
                                 size,
                             )
-                            should_update = True
-                            break
-                if should_update:
-                    to_update.append(package_name)
+                            return False
+            return True
+
+        to_update = []
+        with ThreadPoolExecutor(max_workers=IOWORKERS) as executor:
+            futures = {
+                executor.submit(is_consistent, package_name): package_name
+                for package_name in package_names
+            }
+            try:
+                for future in tqdm(
+                    as_completed(futures),
+                    total=len(package_names),
+                    desc="Checking consistency",
+                ):
+                    package_name = futures[future]
+                    try:
+                        consistent = future.result()
+                        if not consistent:
+                            to_update.append(package_name)
+                    except Exception:
+                        logger.warning(
+                            "%s generated an exception", package_name, exc_info=True
+                        )
+                        raise
+            except:
+                logger.info("Get an exception, exiting...")
+                for future in futures:
+                    future.cancel()
+                sys.exit(1)
+
         logger.info("%s packages to update in check_and_update()", len(to_update))
         return self.parallel_update(to_update, prerelease_excludes)