How can I change default backup location for oracle11g using RMAN?
Some things which might help:

Don't backup into a single file.

allocate CHANNEL d1 DEVICE TYPE disk maxpiecesize 32768M format some_backup_path


Use as many channels as you can stand.

allocate CHANNEL d2 DEVICE TYPE disk maxpiecesize 32768M format some_backup_path

allocate CHANNEL d3 DEVICE TYPE disk maxpiecesize 32768M format some_backup_path

allocate CHANNEL d4 DEVICE TYPE disk maxpiecesize 32768M format some_backup_path


RMAN parallelizes based on the number of channels you allocate; each channel takes part of the work to do.

Have faster disks - both on the system being backed up AND on the system to which backups are being written.  Are you backing up the database over the network?  If so, you're limited by the transmission speed of the network and competing network traffic - probably database-related activity.