SSRS 2016 running slow with blocking

SSRS 2016 - Standard Default Settings. Work Horse for Reporting. Some large reports can run 5 hours+. Most reports cannot be optimized/tuned - 3rd party vendor. On occasion noticed procs "WriteLockSession,CreateChunkSegment,ReadChunkSegment" in multiple sessions with millions of reads and writes , and at times Blocking. Noticed these table have over 400k records: SegmentedChunk, ChunkSegmentMapping and Segment. Is there a standard process to clean up these tables (unused data) ,or check these tables for orphan records? Does 400k sound high (like a problem), have other SSRS instances with much lower counts. Restarted SSRS - no change. Shrink db - no change. Index optimization and update stats - regular maintenance.