Cached, PHP generated Thumbnails load slowly

First, using those multiple domains requires several DNS lookups. You'd be better off combining many of those images into a sprite instead of spreading the requests.

Second, when I load your page, I see most of the blocking (~1.25s) on all.js. I see that begins with (an old version of) jQuery. You should reference that from the Google CDN, to not only decrease load time, but potentially avoid an HTTP request for it entirely.

Specifically, the most current jQuery and jQuery UI libraries can be referenced at these URLs (see this post if you're interested why I omitted the http:):

//ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js//ajax.googleapis.com/ajax/libs/jqueryui/1.8.9/jquery-ui.min.js

If you're using one of the default jQuery UI themes, you can also pull its CSS and images off the Google CDN.

With the jQuery hosting optimized, you should also combine awmlib2.js and tooltiplib.js into a single file.

If you address those things, you should see a significant improvement.

I had a similar problem a few days ago & i found head.js.It's a Javascript Plugin which allows you to load all JS files paralell.Hope that helps.

I am far from an expert but...

In regards to this:"An If-Modified-Since conditional request returned the full content unchanged."and my comments.

The code used to generate the Thumbnails should be checking for the following:

1. Is there a cached version of the thumbnail.
2. Is the cached version newer than the original image.

If either of these are false the thumbnail should be generated and returned no matter what. If they are both true then the following check should be made:

1. Is there a HTTP_IF_MODIFIED_SINCE header
2. Is the cached version's last modified time the same as the HTTP_IF_MODIFIED_SINCE

If either of these are false the cached thumbnail should be returned.

If both of these are true then a 304 http status should be returned. I'm not sure if its required but I also personally return the Cache-Control, Expires and Last-Modified headers along with the 304.

In regards to GZipping, I've been informed that there is no need to GZip images so ignore that part of my comment.

Edit: I didn't notice your addition to your post.

session_cache_limiter('public');header("Content-type: " . $this->_mime);header("Expires: " . gmdate("D, d M Y H:i:s", time() + 2419200) . " GMT");// I'm sure Last-Modified should be a static value. not dynamic as you have it here.header("Last-Modified: " . gmdate("D, d M Y H:i:s",time() - 404800000) . " GMT");

I'm also sure that your code needs to check for the HTTP_IF_MODIFIED_SINCE header and react to it. Just setting these headers and your .htaccess file won't provide the required result.

I think you need something like this:

$date = 'D, d M Y H:i:s T'; // DATE_RFC850$modified = filemtime($filename);$expires = strtotime('1 year'); // 1 Yearheader(sprintf('Cache-Control: %s, max-age=%s', 'public', $expires - time()));header(sprintf('Expires: %s', date($date, $expires)));header(sprintf('Last-Modified: %s', date($date, $modified)));header(sprintf('Content-Type: %s', $mime));if(isset($_SERVER['HTTP_IF_MODIFIED_SINCE'])) {    if(strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) === $modified) {        header('HTTP/1.1 304 Not Modified', true, 304);        // Should have been an exit not a return. After sending the not modified http        // code, the script should end and return no content.        exit();    }}// Render image data