Large I/O efficiency estimates the number of useful pages brought in by each large I/O. For examples, if the value is.5, a 16K I/O returns, on average, 4 2K pages needed for the query, and 4 other pages, either empty pages or pages that share the extent due to lack of clustering.
Low values are an indication that re-creating indexes or running reorg rebuild could improve I/O performance.
