Xcode NSMutableData replaceBytesInRange CPU usage

I am writing an Mp3 player. I am downloading mp3 files from server. Files are encrypted. Encrypt method of my customer is very easy. They just insert a single bit per X byte. Like 323 in code. But decryption process take very long time. And CPU usage is huge? Why? And what can I do?

_mappedData = [[NSMutableData dataWithMappedContentsOfFile:_cachedPath] mutableCopy];

  Singleton *singleton = [Singleton sharedSingleton];

  NSLog(@"Decryption started");

  if(singleton.playerMode == PLAYERMODELISTEN)
  {
     int i = 323;
     while (i < [_mappedData length]) {
         [_mappedData replaceBytesInRange:NSMakeRange(i, 1) withBytes:NULL length:0];
         //CFDataDeleteBytes((CFMutableDataRef)_mappedData, CFRangeMake(i, 1000));
         i += 323;
     }
  }


  [General dismissGlobalHUD];

  NSLog(@"Decryption completed");

Finally i solve that issue myself.

NSMutableData *decryptedData = [[NSMutableData alloc] init];

  NSUInteger length = [_mappedData length];
  NSUInteger chunkSize = 500;
  NSUInteger offset = 0;

  do {
      NSUInteger thisChunkSize = length - offset > chunkSize ? chunkSize : length - offset;
      NSData* chunk = [NSData dataWithBytesNoCopy:(char *)[_mappedData bytes] + offset
                                           length:thisChunkSize
                                     freeWhenDone:NO];
      offset += (thisChunkSize + 1);

      [decryptedData appendData:chunk];

  } while (offset < length);

  _mappedData = decryptedData;

That's my solution. Working very fast.