Review estimates compared to actual delivery
Using Atlassian JIRA - Create a report showing each assignees JEI (JIRA Estimate Index). The JEI is calculated using the original time estimate divided by the logged hours on that issue. A number < 1 = More time spent than estimated. A number > 1 = Less time needed than estimated. You can dynamically add/remove epics in the report to see how that affects the JEI of each assignee. An epic with a uniform low JEI value can be an indication of a overly complex issues or poor acceptance criteria.