Is it all of a sudden or We had the same performance but now we want to improve the performance ??

what optimizer_mode u r using ??
when was the last time you had analyzed all tables and indexes
did you see the sql statements which has high disk read and buffer_gets ?? are they using the indexes??
any of the setting has been changed lately
sort_area_size/db blocks /log buffer
How long(days) has the database running so far