如何让文件下载速度变慢？

This is my final download page of my website where general public is able to download govt documents. From server my code is reading the to-be-downloaded-file and in a loop sending to the client browser.

$fp = fopen($file, "rb");
    while (!feof($fp))
    {
        echo fread($fp, 65536);
        flush(); // this is essential for large downloads
    }
    fclose($fp);
    exit;

I want to send the file very slowly -- that is can I use Sleep function (or anything like that) within this loop and by how much maximum without causing the user client browser to timeout? So that the user gets sufficient time to read the ads displayed on the page while he/she awaits for the file download to finish.

Also I'm not proficient with PHP environment.

(Pl. fogive me for the morality/immorality of this).

You can use bandwidth sharing if you're willing to do this at the Apache level.