Thanks for the guidance, I worked with developer to trace the Toad session and below are the findings
while connecting to PDB(pluggable) Oracle Database, there is a package (not DB stored package) which is being executed for every session whenever user connects from TOAD.
While debugging , developer has found that ALL_SYNONYMS is used in the package where it is taking long time to execute, and that block is being errored out with "ORA-03113 end-of-file on communication channel".
select count(*) from sys.all_synonyms; <<=== This took 8 mins from the back end session, hence culprit for toad connection to be slow
Doc ID 2297471.1 - Queries Against SYS.ALL_SYNONYMS Inside A PDB Are Very Slow
Doc ID 2210513.1 - SQLLDR Utility is Running Slow when ALL_SYNONYMS are Used/Slow ALL_SYNONYMS in 12c CDB
Doc ID 2297471.1 is applicable in my case, after disable the hidden parameter "_common_data_view_enabled", query to sys.all_synonym was coming back quick and toad connection are threw without any delay.
Note - super user login via Toad is quite as it uses DBA_SYNONYMS, users which doesn't have grants to dba_synonyms uses all_synonyms and take long way to login due the issue unless fix given in the note id is not applied
Thanks for the support