Using the Task Board in the Agile view, the "All Assignees" dropdown is slow (couple of seconds) to populate the list. Staying in the same view, a second request for the dropdown is quick. If the project is changed and the same dropdown link clicked, it is again slow to populate.
This suggests the lack of a backend system cache of the Assignees for Greenhopper use.
The official documentation https://confluence.atlassian.com/display/GH/Using+Planning+Board+Modes#UsingPlanningBoardModes-Assignee provides an unhelpful suggested approach.