Strange performance issue

Hi,

On an single Oracle instance 19.6 standard Edition on redhat 7.8, I'm analysing a performance issue and I can't figure why I'm having such results:

I created a simple sql script where I flush both the shared pool and buffer cache. The SQL I'm trying to analyze is an insert into a global temporary table (ON COMMIT PRESERVE) from a SELECT (very big select).

I want to test the effect of changing some parameters and see how the plan change and what is the impact on the speed.

Here's the thing I can't explain:

When I execute the script the first time, it takes 7 min