Have you gone through all the performance tuning recommendations for RMAN ?
A nice presentation with steps here http://docshare04.docshare.tips/files/21043/210437276.pdf
In particular, it talks about identifying what the raw performance of your disk infrastructure is. If that's slow, then RMAN can't magically make it better.
If its not slow *and* you've been through *al* of the recommendations, then its time to talk to Support