Faster way i Know for thumbnail (low quality)

Calculate byte length for PixelFormat

private static byte BytesPerPixel(PixelFormat px)
        {
            switch (px)
            {
                case PixelFormat.Format16bppRgb555:
                case PixelFormat.Format16bppRgb565:
                case PixelFormat.Format16bppGrayScale:
                case PixelFormat.Format16bppArgb1555: return 2;
                case PixelFormat.Format24bppRgb: return 3;
                case PixelFormat.Format32bppRgb: return 4;
                case PixelFormat.Format32bppArgb: return 4;
                case PixelFormat.Format32bppPArgb: return 4;
            }
            return 0;
        }

Copy to resized Image

public static Bitmap ThumbnailLow(Bitmap img, Size sz)
        {
            if (img == null || sz.IsEmpty) return null;
            if (sz.Width > img.Width) sz = new Size(img.Width, sz.Height);
            if (sz.Height > img.Height) sz = new Size(sz.Width, img.Height);
            PixelFormat px = img.PixelFormat;
            PixelFormat pxn = PixelFormat.Format16bppRgb565;
            if (px == PixelFormat.Format32bppArgb || px == PixelFormat.Format32bppPArgb ||
                px == PixelFormat.Format32bppRgb) pxn = PixelFormat.Format24bppRgb;
            byte bytesPerPixel = BytesPerPixel(px), bytesPerPixel2 = BytesPerPixel(pxn);
            Bitmap nueva = new Bitmap(sz.Width, sz.Height, pxn);
            BitmapData bmpData = img.LockBits(new Rectangle(0, 0, img.Width, img.Height), ImageLockMode.ReadWrite, px);
            BitmapData bmpData2 = nueva.LockBits(new Rectangle(0, 0, sz.Width, sz.Height), ImageLockMode.ReadWrite, pxn);
            //double less speed
            float inc_djn = img.Width / (float)sz.Width;
            float inc_din = img.Height / (float)sz.Height;
            float din = 0, djn = 0;
            bool _16bits = bytesPerPixel == 2 || bytesPerPixel2 == 2;
            unsafe
            {
                byte* ptr = (byte*)(bmpData.Scan0);
                byte* ptr2 = (byte*)(bmpData2.Scan0);
                int nOffset = bmpData.Stride - (bmpData.Width * bytesPerPixel);
                int nOffset2 = bmpData2.Stride - (bmpData2.Width * bytesPerPixel2);
                int h = bmpData.Height, w = bmpData.Width;
                for (int i = 0; i < h; i++)
                {
                    bool lok = i >= din;
                    djn = 0;
                    for (int j = 0; j < w; j++)
                    {
                        if (lok && j >= djn)
                        {
                            ptr2[0] = ptr[0];
                            ptr2[1] = ptr[1];
                            if (!_16bits) ptr2[2] = ptr[2];
                            ptr2 += bytesPerPixel2;
                            djn += inc_djn;
                        }
                        ptr += bytesPerPixel;
                    }
                    if (lok) { ptr2 += nOffset2; din += inc_din; }
                    ptr += nOffset;
                }
            }
            img.UnlockBits(bmpData);
            nueva.UnlockBits(bmpData2);
            return nueva;
        }
        

Test! (in debug mode)

//copy from screen
            Rectangle rc = Screen.PrimaryScreen.Bounds;
            Bitmap img = new Bitmap(rc.Width, rc.Height, PixelFormat.Format16bppRgb555);
            using (Graphics memoryGrahics = Graphics.FromImage(img))
                memoryGrahics.CopyFromScreen(rc.X, rc.Y, 0, 0, rc.Size, CopyPixelOperation.SourceCopy);
            //img size 1280x1024
            Stopwatch sw = new Stopwatch(), sw2 = new Stopwatch();
            
            //non-proportional
            sw.Start();
            Image i1 = ThumbnailLow(img, new Size(100, 100)); sw.Stop();// 7 ms
            sw2.Start();
            Image i2 = img.GetThumbnailImage(100, 100, null, IntPtr.Zero); sw2.Stop();//15 ms
            //proportional
            sw.Reset(); sw.Start();
            i1 = ThumbnailLow(img, new Size(128, 102)); sw.Stop();// 6 ms
            sw2.Reset(); sw2.Start();
            i2 = img.GetThumbnailImage(128, 102, null, IntPtr.Zero); sw2.Stop();//15 ms
            

For img size 1280x1024 ThumbnailLow : 6 to 7 ms GetThumbnailImage: 15 ms Pros.... the quality is the same for videos or images in both cases Opposites.... in text mode... becomes hard to read if you want speed this is your method, but if you wants qualitty definitely not