This indicates that rows are spanning across multiple blocks. While this may simply be indicative of long rows, it can also be indicative of data fragmentation.
Recommendation: Consider reorganizing data via export/import to pack rows more tightly and alleviate chained row issues. Another option is using larger block sizes on tables with long rows.
It has been seen (anecdotal evidence) that dropping and recreating Global Temporary Tables on a periodic basis can have positive performance improvements.
Additionally it has been proven that issuing REBUILDs on indexes to compact it and minimize fragmented space can help performance.
Recommendation: Perform REBUILDs on indexes. After issuing a REBUILD on your indexes be sure to gather stats via the DBMS_STATS package. Also consider reorganizing data via export/import to pack rows more tightly and alleviate chained row issues.
(FRAGMENTATION = POOR PERFORMANCE)