Look at the data source for the report to find what is being executed. There are several different ways to capture or find the execution plan. This article goes through the different ways you can do that:
Retrieving SQL Server Query Execution Plans
You may also want to look at the ExecutionLog3 view in the ReportServer database. TimeDataRetrieval is for interacting with the data source, extensions which would mean you want to check the query for the data source. High times in other areas could indicate a different issue so it would be good to check. This article is for the older ExecutionLog3 but still goes over the same columns and what the different areas are measuring:
ExecutionLog2 View – Analyzing and Optimizing Reports
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