diff --git a/PythonForVideoMemos/VMPython/VMPythonResourceDownloader/VMPythonResourceDownloader.m b/PythonForVideoMemos/VMPython/VMPythonResourceDownloader/VMPythonResourceDownloader.m
index 2bfa535..7d03e4a 100644
--- a/PythonForVideoMemos/VMPython/VMPythonResourceDownloader/VMPythonResourceDownloader.m
+++ b/PythonForVideoMemos/VMPython/VMPythonResourceDownloader/VMPythonResourceDownloader.m
@@ -56,8 +56,8 @@ @implementation VMPythonResourceDownloader
 
 - (void)dealloc
 {
-  if (self.downloadingOperationQueue) {
-    [self.downloadingOperationQueue cancelAllOperations];
+  if (_downloadingOperationQueue) {
+    [_downloadingOperationQueue cancelAllOperations];
   }
 }
 
@@ -82,6 +82,19 @@ - (instancetype)init
 
 #pragma mark - Getter
 
+- (NSOperationQueue *)downloadingOperationQueue
+{
+  if (!_downloadingOperationQueue) {
+    _downloadingOperationQueue = [[NSOperationQueue alloc] init];
+    _downloadingOperationQueue.maxConcurrentOperationCount = 1;
+    _downloadingOperationQueue.suspended = self.suspended;
+    _downloadingOperationQueue.qualityOfService = NSQualityOfServiceUtility;
+  }
+  // Set `suspended=YES` if want to pause all operations temporary.
+  //self.downloadingOperationQueue.suspended = YES;
+  return _downloadingOperationQueue;
+}
+
 - (NSCharacterSet *)invalidFilenameCharacterSet
 {
   if (!_invalidFilenameCharacterSet) {
@@ -221,9 +234,7 @@ - (void)setSuspended:(BOOL)suspended
 {
   _suspended = suspended;
   
-  if (self.downloadingOperationQueue) {
-    self.downloadingOperationQueue.suspended = self.suspended;
-  }
+  self.downloadingOperationQueue.suspended = self.suspended;
 }
 
 #pragma mark - Public
@@ -308,14 +319,6 @@ - (NSString *)downloadWithURLString:(NSString *)urlString
                       preferredName:(NSString *)preferredName
                            userInfo:(NSDictionary *)userInfo
 {
-  if (!self.downloadingOperationQueue) {
-    self.downloadingOperationQueue = [[NSOperationQueue alloc] init];
-    self.downloadingOperationQueue.maxConcurrentOperationCount = 1;
-    self.downloadingOperationQueue.suspended = self.suspended;
-  }
-  // Set `suspended=YES` if want to pause all operations temporary.
-  //self.downloadingOperationQueue.suspended = YES;
-  
   VMPythonDownloadOperation *operation = [[VMPythonDownloadOperation alloc] initWithURLString:urlString
                                                                                      inFormat:format
                                                                                 totalFileSize:totalFileSize