After upgrade to 12c we experience the excessive CPU usage - during weekend it was not a problem, but on Monday after the upgrade the application based on that db was slow, very, very slow.
The workaround was switching the optimizer into the 11.2.0.4 mode with optimizer_feature_enable. The main suspect is Adaptive Optimizer features so possible one could disable it with optimizer_adaptive_feature set to false, but not tested yet.
