There are so many variables with respect to Oracle performance. First, start with:

http://hojohn.photo.net/ora81/DOC/server.815/a67775/toc.htm

The complete index can be found at:

http://hojohn.photo.net/ora81/DOC/server.815/

The problem could be with the design of the SPs as well. Without knowing what the SPs are doing, this is almost impossible to answer. The problem could too many or too few indexes as well. With each insert or change in index key, every index on each table being updated also gets updated. If you have too few indexes, then optimization becomes a problem.