How to increase performance on RMAN

Hi All;

I have rman backup, it ran 18 hrs to backup 3.4TB in last 2 weeks but now it takes 25hrs to backup same size (3.4TB),

If I set number of RMAN Channels = 1 and Fileperset = 1 in dataset then the output is:

2019-12-26 10:18:53 avtar Info <6083>: Backed-up 8.000 GB in 3.06 minutes: 157 GB/hour (20 files/hour)

If I set number of RMAN Channels = 1 and Fileperset = 2 in dataset then the output is:

2019-12-24 13:36:00 avtar Info <6083>: Backed-up 11.40 GB in 22.68 minutes: 30 GB/hour (3 files/hour)

If I set number of RMAN Channels = 1 and Fileperset = 4 in dataset then the output is:

2019-12-18 11:01:41 avtar Info <6083>: Backed-up 19.28 GB in 31.46 minutes: 37 GB/hour (2 files/hour)

Any suggestion on it , please ? Thank you.