12.2 : Execute vs Fetch => SQL Developer

Hello,

A user complains about a query (SELECT) which takes 15 min instead of 5 sec usually.

I re-executed the query on SQL Developer (Oracle 12.2).

Here is the behavior of the query execution :

1 min to display the first 50 rows (SQL Developer Configuration).

15 min to display the rest of the rows  (800 in total) by doing CONTROL-A in SQL Developer results window

Is this normal behavior?

Is this a performance issue at runtime or fetch level or both?

Thank you.

GD.