While backing up to the Oracle Fast Recovery Area (FRA), you determined the backup istaking...

If an RMAN BACKUP VALIDATE command takesroughly the same time as an actual backup, then both read and write I/O are likely bottlenecks.

Setting DBWR_IO_SLAVES to a non zero value can improve backup performance when using synchronous I/O.

If an RMAN BACKUP VALIDATE command takes noticeably lessthan an actual backup, then write I/O is a likely bottleneck.

If an RMAN BACKUP VALIDATE command takesroughly the same time as an actual backup, then read I/O is a likely bottleneck.

Data files with a high value in V$BACKUP_SYNC_IO.DISCRETE_BYTES_PER_SECOND are a potential performance bottleneck when synchronous I/O is used.

Setting DBWR_IO_SLAVES to a non zero value can improve backup performance when using asynchronous I/O/

Data files with a high value in V$BACKUP_ASYNC_IO.SHORT_WAITS are a potential performance bottleneck when asynchronous I/O is used.