缩略图生成代码，批量生成太慢，求改进方法（有代码）

using System;
using System.Data;
using System.Configuration;
using System.Drawing;
using using System.Drawing.Drawing2D;
/// <summary>
/// MakeThumbnails 的摘要说明
/// </summary>
public class MakeThumbnails
{
    public MakeThumbnails()
    {
    }
    /// <summary>
    /// 生成缩略图(最终图片固定大小,图片按比例缩小,并为缩略图加上边框,以jpg格式保存)
    /// </summary>
    /// <param name="sourceImg">原图片(物理路径)</param>
    /// <param name="toPath">缩略图存放地址(物理路径)</param>
    /// <param name="width">缩略图宽度</param>
    /// <param name="height">缩略图高度</param>
    /// <param name="backColor">如果图片按比例缩小后不能填充满缩略图,则使用此颜色填充(比如"#FFFFFF")</param>
    /// <param name="borderColor">边框颜色(比如"#999999")</param>
    public static void MakePic(string sourceImg, string toPath, int width, int height, string backColor, string borderColor)
    {
        string[] files = Directory.GetFiles(sourceImg, "*.JPG");
        string[] toPathFilename = new string[files.Length];
        int[] nArray = new int[files.Length];

        string mode;
        int i = -1;
        //for (int i = 0; i < files.Length; i++)
        foreach (int j in nArray)
        {
            i++;
            System.Drawing.Image originalImage = System.Drawing.Image.FromFile(files[i]);
            int towidth = width;
            int toheight = height;
            int x = 0;
            int y = 0;
            int ow = originalImage.Width;
            int oh = originalImage.Height;
            toPathFilename[i] = files[i].Substring(files[i].LastIndexOf(@"\") + 1);

            if (ow < towidth && oh < toheight)
            {
                towidth = ow;
                toheight = oh;
            }
            else
            {
                if (originalImage.Width / originalImage.Height >= width / height)
                {
                    mode = "W";
                }
                else
                {
                    mode = "H";
                }
                switch (mode)
                {
                    case "W"://指定宽，高按比例
                        toheight = originalImage.Height * width / originalImage.Width;
                        break;
                    case "H"://指定高，宽按比例
                        towidth = originalImage.Width * height / originalImage.Height;
                        break;
                    default:
                        break;
                }
            }
            //新建一个bmp图片
            System.Drawing.Image bitmap = new System.Drawing.Bitmap(width, height);

            //新建一个画板
            System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitmap);
            //设置默认插值法
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.Low;
            ////设置高速度呈现平滑程度
            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighSpeed;
            g.PixelOffsetMode = PixelOffsetMode.HighSpeed;
            
            //清空画布并以指定颜色填充
            g.Clear(ColorTranslator.FromHtml(backColor));
            //在指定位置并且按指定大小绘制原图片的指定部分
            int top = (height - toheight) / 2;
            int left = (width - towidth) / 2;
            g.DrawImage(originalImage, new System.Drawing.Rectangle(left, top, towidth, toheight),
            new System.Drawing.Rectangle(x, y, ow, oh), System.Drawing.GraphicsUnit.Pixel);
            Pen pen = new Pen(ColorTranslator.FromHtml(borderColor));
            g.DrawRectangle(pen, 0, 0, width - 1, height - 1);
            try
            {
                //以jpg格式保存缩略图
                bitmap.Save(toPath + "\\" + toPathFilename[i], System.Drawing.Imaging.ImageFormat.Jpeg);
            }
            catch (System.Exception e)
            {
                throw e;
            }
            finally
            {
                if (i % 3 == 0)
                {
                    originalImage.Dispose();
                    bitmap.Dispose();
                    g.Dispose();
                }
               
            }
        }

    }
}