Summary
The following script lists the contents of x$kcbfwait, I have eliminate files where there have been no waits.
SELECT indx+1 FILE#, d.file_name, COUNT, TIME, TIME/(DECODE(COUNT,0,1,COUNT)) AVG
FROM x$kcbfwait, dba_data_files D
WHERE indx < (SELECT COUNT(*) FROM v$datafile)
AND indx+1 = D.file_id
AND TIME > 0
ORDER BY AVG DESC;