RMAN backup is very slow

 Today my RMAN cold back was very slow. It should take around 75 mins. But it took almost 200 mins. How do I find why this was slow today? our next cold backup is on sunday.

We use control file as the repository and my database version is 9.2.0.8

When I execute the following query now, I see:

  SELECT p.SPID, sw.EVENT, sw.SECONDS_IN_WAIT AS SEC_WAIT, sw.STATE, CLIENT_INFO
 FROM V$SESSION_WAIT sw, V$SESSION s, V$PROCESS p
  WHERE (sw.EVENT LIKE 'sbt%' or s.client_info like '%rman%') AND s.SID=sw.SID