Data visualization consultant handle large datasets by using robust tools like Power BI, Tableau, and R or Python-based visualization libraries. They first clean and transform the data to ensure accuracy, using methods like aggregation or filtering to focus on the most relevant information. Consultants design data models to optimize performance and avoid overwhelming the user with excessive details. They may also implement techniques like sampling or data partitioning to manage volume, ensuring that visualizations load quickly and provide meaningful insights without sacrificing accuracy or comprehensiveness.