Cache results of CPU count detection

src/calibre/__init__.py

@@ -469,8 +469,11 @@ class CurrentDir(object):
             pass
 
 
+_ncpus = None
 def detect_ncpus():
     """Detects the number of effective CPUs in the system"""
+    global _ncpus
+    if _ncpus is None:
         if iswindows:
             import win32api
             ans = win32api.GetSystemInfo()[5]
@@ -482,7 +485,8 @@ def detect_ncpus():
             except Exception:
                 from PyQt5.Qt import QThread
                 ans = QThread.idealThreadCount()
-    return max(1, ans)
+        _ncpus = max(1, ans)
+    return _ncpus
 
 
 relpath = os.path.relpath