RMAN backups of SAP database

The Basis admin from another team has asked if I can help him find out why RMAN backups of his SAP database cause severe performance issues in the database with only two channels. Yet the SAP backup method runs with 6 parallel processes in about 4-5 hours. I know nothing about how the SAP backups are configured and don't have access to any of that information, so I'm not sure you'll be able to provide much help to us. I do know the following:

• they don't use ASM
• all files are on the same NFS mount, including the datafiles, binaries, logs, audit files, etc.
• archivelogs are kept for 3 days and written to another NFS mounted drive
• SAP backup method uses 6 channels, backup completes in 4-5 hours
• RMAN backup was set for 2 channels, backup tool over 12 hours and caused significant performance degradation (so I was told, I can't prove it)

Any suggestions on what they should look at to see what's going on or adjustments they should look into? I've looked at their RMAN configuration and it's actually pretty close to what I use in my production databases, just minor differences for path names and the like.