RMAN – V$BACKUP_ASYNC_IO

SELECT TYPE,TO_CHAR(OPEN_TIME,’dd/mm/yyyy hh:mi’),TO_CHAR(CLOSE_TIME,’dd/mm/yyyy hh:mi’), SHORT_WAITS,LONG_WAITS,IO_COUNT,LONG_WAITS/IO_COUNT,effective_bytes_per_second, FILENAME
FROM V$BACKUP_ASYNC_IO

You can check the RMAN CHANNEL PERFORMANCE using this query