The Code Assist experience has poor performance in Toad for Oracle 12.6.
At times, it’s so bad that it's unusable.
After typing a dot (.) after the table name, Toad hangs for several minutes and doesn't populate a pick list as expected.
There are no issues like this in version 12.1.
Revert back to Toad for Oracle 12.1.
This has been currently fixed in the beta after version 188.8.131.52.
Waiting for fix in a future version of Toad for Oracle.