JQL does not appear to short-circuit evaluation which causes more issues to be considered than necessary. We recently noticed how impactful this is using "was" because it queries the database and not the index. Consider:

project = ABC AND status was Resolved

In our observation this seems to evaluate to all resolved issues in the entire instance, not just those in project ABC. JQL would evaluate much faster if it took the result of each clause and reduced the scope of issues that the next clause was evaluated against.