How to Identify Asynchronous I/O Bottlenecks

You can use V$BACKUP_ASYNC_IO to monitor the performance of asynchronous I/O, the file in this dynamic performance view field
with the largest ratio of LONG_WAITS to IO_COUNT may be where there is an asynchronous I/O bottleneck.

IO_COUNT is the number of I/Os to be performed on the file

LONG_WAITS is the number of occurrences of "the backup/restore process instructed the operating system to wait for the I/O to complete".Async I/O bottlenecks are completely avoided with zero wait times.