zkLoader outputs html which references zk.wcs, zk.wpd and zul.lang.wpd. These are given header to encourage them to cache:

HTTP/1.1 200 OK
Cache-Control: public, max-age=31536000
Content-Encoding: gzip
Content-Language: en-US
Content-Type: text/javascript;charset=UTF-8
Expires: Sun, 09 Dec 2012 19:14:22 GMT
Last-Modified: Sat, 10 Dec 2011 19:04:32 GMT
Server: Jetty(7.x.y-SNAPSHOT)
Content-Length: 98783
Connection: keep-alive

Yet when I hit the page repeatedly with either Google Chrome or Firefox the developer consoles show that the browser is refetching the data. This is adding up to 0.6 seconds of page load time due to the size of those resources.

Test Page: http://glowing-light-1070.herokuapp.com/zktodo_d.zul
Test page source code: https://github.com/simbo1905/ZkToDo2/blob/master/src/main/webapp/zktodo_d.zul

[Heroku cloud swaps out the app if it is idle; please wait many seconds first load].

I get the same behavior running on a local jetty server (mvn jetty:run).