diff --git a/Lib/importlib/_bootstrap.py b/Lib/importlib/_bootstrap.py
--- a/Lib/importlib/_bootstrap.py
+++ b/Lib/importlib/_bootstrap.py
@@ -19,6 +19,7 @@ work. One should use importlib as the pu
 
 # Bootstrap-related code ######################################################
 
+import time
 CASE_INSENSITIVE_PLATFORMS = 'win', 'cygwin', 'darwin'
 
 
@@ -764,18 +765,28 @@ class _FileFinder:
         self._path_cache = set()
         self._relaxed_path_cache = set()
         self._cache_refresh = 0
+        self._last_time = -1.0
 
     def find_module(self, fullname):
         """Try to find a loader for the specified module."""
         tail_module = fullname.rpartition('.')[2]
-        try:
-            mtime = _os.stat(self.path).st_mtime
-        except OSError:
-            mtime = -1
-        if mtime != self._path_mtime or _cache_refresh != self._cache_refresh:
+        invalidate_cache = False
+        if _cache_refresh != self._cache_refresh:
+            invalidate_cache = True
+            self._cache_refresh = _cache_refresh
+        else:
+            now = time.time()
+            if now >= self._last_time + 1.0:
+                self._last_time = now
+                try:
+                    mtime = _os.stat(self.path).st_mtime
+                except OSError:
+                    mtime = -1
+                if mtime != self._path_mtime:
+                    invalidate_cache = True
+                    self._path_mtime = mtime
+        if invalidate_cache:
             self._fill_cache()
-            self._path_mtime = mtime
-            self._cache_refresh = _cache_refresh
         # tail_module keeps the original casing, for __file__ and friends
         if _relax_case():
             cache = self._relaxed_path_cache