org.zkoss.zel.ExpressionFactory is currently not caching the class to create a newInstance from. Under high request volume that leads to frequent class loading, when ExpressionFactory is created often.

ZK8 already contains improvements for this caching the ExpressionFactory
https://github.com/zkoss/zk/blob/ec0d00ea125d6bcb8b5c27bf6ca1bcebf567947d/zel/src/org/zkoss/zel/ExpressionFactory.java#L139

The older ZK versions would benefit equally from that improvement.