It seems to me that the "Import Tables" functionality (in "Tools" menu) uses simple INSERTs instead of "bulk" INSERTs. Is there any "bulk" DML functionality in OCI at all or it's available only in PL/SQL?
OCI does have bulk import functionality, but it's not directly exposed in PL/SQL Developer (only indirectly thru Oracle's imp utility via the Import Tables tool).
I have added this to the list of enhancement requests.