I'm using a QFileIconProvider class to show image thumbnails in a QFileDialog by overriding the 'icon' method to load from a file to a QIcon.

After implementing this, the file dialog is very slow in navigating through sub-folders and hangs for a few seconds before changing directory.

C++ class is below.

Why is the rendering being slow and how can I speed it up?

Qt Code: 
  1. QIcon CFileIconProvider::icon(const QFileInfo &info) const
  2. {
  3.     const QString &path = info.absoluteFilePath();
  4.  
  5.     // check if 'info' is a drive type
  6.     if (path.isEmpty())
  7.         return QFileIconProvider::icon(QFileIconProvider::Drive);
  8.  
  9.     if (info.isFile()) {
  10.         if (QFile(info.absoluteFilePath()).exists()) {
  11.             // show file as icon
  12.             QImage originalImage(info.absoluteFilePath());
  13.  
  14.             if (!originalImage.isNull())
  15.                 return QIcon(info.absoluteFilePath());
  16.         }
  17.     }
  18.  
  19.     // check if 'info' is a directory type
  20.     if (info.isDir())
  21.         return QFileIconProvider::icon(QFileIconProvider::Folder);
  22.  
  23.     return QIcon();
  24. }
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Qt Code: 
  1. #ifndef FILEICON_PROVIDER_H
  2. #define FILEICON_PROVIDER_H
  3.  
  4. #include <QFileIconProvider>
  5. #include <QObject>
  6. #include <QIcon>
  7.  
  8. class CFileIconProvider : public QFileIconProvider
  9. {
  10.  
  11. public:
  12.     CFileIconProvider();
  13.     QIcon icon(const QFileInfo &info) const override;
  14.  
  15. };
  16.  
  17. #endif // FILEICON_PROVIDER_H
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