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It's impossible to answer this question in a few sentences. Your best bet
is to consult one of the excellent performance tuning books currently on the
market. I particulary recommend Cary Millsap and Jeff Holt's Optimizing
Oracle performance, and Oracle performance tuning 101, by Vaidyanatha,
Deshpande and Kostelac.
Briefly, though, the most common causes of Oracle performance problems are
applications that cause the database engine to do much more work than is
necessary to achieve the desired result; poorly-designed database schemas;
and poorly-designed databases. These can manifest themselves in many ways:
excessive CPU consumption due to too many logical I/Os (visits to database
blocks in memory), excessive disk reads due to missing indexes, or excessive
contention for shared resources.
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