The "Space Documents Panel" makes the Panel Wizard slow on wikis with many users

Steps to reproduce:

• create > 10k of dummy users using a snippet such as http://snippets.xwiki.org/xwiki/bin/view/Extension/Create+200+Dummy+Users
• Open the Panel Wizard from the wiki Administration

Results:

• the Panel Wizard will give OOM on the default Jetty/Hsqldb distribution
• the Panel Wizard will take up to 5min on a production wiki (Tomcat, MySQL) with ~13000 entries (11k users + 2k groups)