ASP.NET下载速度限制可以通过以下代码示例来实现：

1. 使用Response对象的AddHeader方法设置下载速度限制。

protected void DownloadFile(string filePath)
{
    Response.ContentType = "application/octet-stream";
    Response.AddHeader("Content-Disposition", "attachment; filename=" + Path.GetFileName(filePath));
    Response.AddHeader("Content-Length", new FileInfo(filePath).Length.ToString());
    Response.AddHeader("Content-Transfer-Encoding", "binary");
    Response.AddHeader("Accept-Ranges", "bytes");
    Response.AddHeader("Cache-Control", "private");
    Response.AddHeader("Pragma", "private");
    Response.AddHeader("Expires", "0");
    
    // 设置下载速度限制为100KB/s
    Response.AddHeader("X-ThrottleBandwidth", "102400");

    using (FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read))
    {
        byte[] buffer = new byte[1024];
        int bytesRead;
        while ((bytesRead = fileStream.Read(buffer, 0, buffer.Length)) > 0)
        {
            Response.OutputStream.Write(buffer, 0, bytesRead);
            Response.Flush();
        }
    }

    Response.End();
}

2. 使用HttpModule来实现下载速度限制。

首先，创建一个名为DownloadSpeedModule的类，实现IHttpModule接口。

using System;
using System.IO;
using System.Web;

public class DownloadSpeedModule : IHttpModule
{
    public void Init(HttpApplication context)
    {
        context.PreSendRequestHeaders += OnPreSendRequestHeaders;
    }

    public void Dispose()
    {
    }

    private void OnPreSendRequestHeaders(object sender, EventArgs e)
    {
        HttpContext context = ((HttpApplication)sender).Context;

        if (context.Request.Url.LocalPath.EndsWith(".zip"))
        {
            // 设置下载速度限制为100KB/s
            context.Response.AddHeader("X-ThrottleBandwidth", "102400");
        }
    }
}

在Web.config文件中注册HttpModule。

这样就可以在下载文件时限制下载速度为100KB/s。