RMAN backup taking a lot longer

Our production incremental RMAN level1 backup was taking the usual 1-2 hours to complete for past couple of years.  All of a sudden now and this past month, it's now taking 8-9 hours to complete.  What can i check for?

On 11.2.0.2 without catalog.  DB is roughly 500GB.

Here is my current RMAN script:

connect target /

run {

allocate channel d1 type disk;

backup

incremental level 1

tag lvl1_db

format '/data/backup/rman/PROD/lvl1_db_%t_%s_p%p'

(database include current controlfile);

delete obsolete;

release channel d1;

}

Should I consider allocating another channel?  If so, can 2 channel write to same disk location?