Steps to Reproduce

See fiddle :
http://zkfiddle.org/sample/26euevb/1-ListboxCeRod
Scroll down the listbox

Actual Result

Items are loaded a few at a time when scrolling.
The maximum size of a loading chunk is preloadSize
This avoid rendering many items in one step, but cause many network transactions.

Expected Result

A minimum or fixed stack size should be available to give the option of loading more items with each transactions.

Debug Info

Current chunck size adjustment:
https://github.com/zkoss/zk/blob/master/zul/src/org/zkoss/zul/Listbox.java#L3709

Current documentation indicates that preloadSize is "the number of items to preload when receiving the rendering request from the client"
https://www.zkoss.org/wiki/ZK_Configuration_Reference/zk.xml/The_Library_Properties/org.zkoss.zul.listbox.preloadSize

Root Cause

Workaround

Item loading can be forced from java side using the renderItems() method on listbox