Hello, i have the same performance problems regarding statements with ALL_SYNONYMS.
I have 126.96.36.199 PSU6 - nothing changes....
My solution to fix this is to use a hint - I know, it is dirty - but it works for me :
CURSOR C_SYNO (PI_OWNER IN VARCHAR2, PI_TABLE IN VARCHAR2) IS
SELECT /*+ optimizer_features_enable('9.2.0') */
So back to 9.2.0 works fine : From 9 minutes to 0.8 seconds. Lookning to the access plane is still awfull....
I have tried with optimizer to 10.2.0 : It does not work...