RMAN backup slow performance issue

Hallo, this is the scenario:

Oracle 11.2.0.4 Standard Edition

OS: RedHat Linux EE 6.4

DB dimension: about 1,6 Tb , RAC + ASM, 120 datafiles

Full backup via RMAN in the last year has become more and more slow, it can also go on for more than 24 hours

I know that, due to Standard Edition, you can only use 1 channel but:

- DB dimension is almost the same since last 8 months

- RMAN script is never changed

so it's not easy to understand what's happened to increase backup duration of about 40-50%.

It's not easy also to tune it, I mean to execute a test changing something in the script, just because rman is almost always running.