See the question and my original answer on StackOverflow

You could use WPF interop and use the DecodePixelWidth/Height properties. They use underlying Windows imaging layer technology ("Windows Imaging Component") to create an optimized thumbnail, saving lots of memory (and possibly CPU): How to: Use a BitmapImage (XAML)

You can also use WPF/WIC by code, with a code like this (adapted from this article The fastest way to resize images from ASP.NET. And it’s (more) supported-ish.. You just need to add a reference to PresentationCore and WindowsBase which shouldn't be an issue for a desktop app.

    // needs System.Windows.Media & System.Windows.Media.Imaging (PresentationCore & WindowsBase)
    public static void SaveThumbnail(string absoluteFilePath, int thumbnailSize)
        if (absoluteFilePath == null)
            throw new ArgumentNullException(absoluteFilePath);

        var bitmap = BitmapDecoder.Create(new Uri(absoluteFilePath), BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.None).Frames[0];
        int width;
        int height;
        if (bitmap.Width > bitmap.Height)
            width = thumbnailSize;
            height = (int)(bitmap.Height * thumbnailSize / bitmap.Width);
            width = (int)(bitmap.Width * thumbnailSize / bitmap.Height);
            height = thumbnailSize;

        var resized = BitmapFrame.Create(new TransformedBitmap(bitmap, new ScaleTransform(width / bitmap.Width * 96 / bitmap.DpiX, height / bitmap.Height * 96 / bitmap.DpiY, 0, 0)));
        var encoder = new PngBitmapEncoder();
        var thumbnailFilePath = Path.ChangeExtension(absoluteFilePath, thumbnailSize + Path.GetExtension(absoluteFilePath));
        using (var stream = File.OpenWrite(thumbnailFilePath))

Otherwise there are lots of tools out there like MagicScaler, FreeImage ImageSharp, ImageMagick, Imazen, etc. Most were written for ASP.NET/Web server scenarios (for which WPF is officially not supported but works, read the article) and are also cross-platform which you don't seem to need. I'm not sure they're generally faster or use less memory than builtin Windows technology, but you should test all this in your context.

PS: otherwise there's no magic bullet, bigger images take more time.