Thank you for your answer. I think that the situation does not change much with the new case.
I am very satisfied with its transparent acrylic case and I have never thought of a solution with a heatsink with or without a fan.
I think it’s Thermal Throttling and the heatsink only creates a thermal inertia and that’s why I did not put it.
now with an ambient temperature of 20 degrees the cpu works in idle at only 43 degrees.
I think I will buy the beautiful case