SQL performance can degrade for many reasons, some of most common are:
– plan changes
– data skewness
– low caching efficiency
– data growth
– contention.
All these factors are relatively well known. A somewhat less common, although not exceptionally rare scenario, is read consistency overhead due to concurrent DML against queried tables. Because of being less common, this scenario is often overlooked, which leads to false diagnoses (and eventually to “fixes” that can do more harm than good).
Oracle Diagnostician 