BRBackup taking long time in Oracle

Hi Experts,

We are having an issue with the ECC Dev system. Full DB online backup is scheduled in DB13 (Oracle database ) in daily basis. Backup is taking much time than previously and after checking the log we found that there is one particular datafile that is taking much time to back up. Initially, the datafile was 30 GB and by HWM setup we reduced it to 10 GB because free space was there. Even it's taking too much time. Can anybody suggest the possible reason for this one?

Thanks ...