the report starts displaying data once there is enough to display. The query will to continue to create the dataset in the background. The behaviour you are describing implies the query is slow.
Suggest open the query and time until the recordcount is displayed at the bottom - this is how long it takes to calculate.
If it is slow, then the only thing you can do is to speed it up. Without knowing the complexity of your query and the structure of your tables it is difficult to advise.
Slow queries may because of a slow network, use of domain functions, use of subqueries, lack of indexing in tables, using * as the initial character in Like criteria, using tables across multiple databases among others