Check in specify maximum limits, you need to adjust the "Read rate (KB/sec) (RATE)" value.
Select Specify maximum limits to access several I/O and backup set limits. The following parameters should only have to be modified on rare occasions. If these values are not changed, the backup uses the default values defined in RMAN. RMAN default values usually provide the best performance.
Maximum I/O Limits parameters:
Read rate (KB/sec) (RATE) specifies the maximum number of kilobytes (KB) that RMAN reads each second on this channel. This parameter sets an upper limit for bytes read so that RMAN does not consume too much disk bandwidth and degrade performance.
Size of backup piece (KB) (MAXPIECESIZE) specifies the maximum size of each backup piece that is created on this channel.
Number of open files (MAXOPENFILES) controls the maximum number of input files that the backup operation can have open at any given time.
Maximum backup set limits parameters:
Number of files per backup set (FILESPERSET) specifies the maximum number of input files to include in each output backup set.
Size of the backup set (KB) (MAXSETSIZE) specifies a maximum size for a backup set in kilobytes.