Bulk Edit assignee list performance limitation due to using CollectionList

From the ticket:

Bulk Edit is broken due to following line of code in getAssigneeOptionsList(BulkEditBean bulkEditBean) method of AssigneeSystemField.java:

users.retainAll(getAssignableUsers(bulkEditBean, issue));

getAssignableUsers(bulkEditBean, issue) method returns Collection <User> object. Therefore, the above code is invoking Collection.retainAll(Collection). Collection.retainAll(Collection) implementations aren't optimized.
When a sufficiently large collection is used as an argument, the performance is pathologically slow. This is a known Issue in JAVA. Please check following bug:

http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5028425

Performance can be optimized by transforming collection argument to HashSet. Cast return value of getAssignableUsers(bulkEditBean, issue) method to HastSet.

private Collection<User> getAssignableUsers(OperationContext operationContext, Issue issue)

Please ask JIRA developers to verify this solution.