rman backup speed single channel vs. multi-channel

I have the following rman backup configuration set.

CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM 4;

While in the actual rman backup script, I didnot explicitly wrote in allocate channel and release channel command. The backup automatically allocates four channel and release those afterwards.

However, our system admin is advising me to change the backup to single channel because he says the multiplexing messes up the NFS deduplication. After change the configuration for a test instance, the backup time tripled.

CONFIGURE DEVICE TYPE DISK BACKUP TYPE TO BACKUPSET PARALLELISM 1;

My question is should the allocate/channel command be explict wrote in the rman backup script? Is the configuration need any modification?