FPS drop in a project after loading again the project

I’m using Editorversion 20022.3.6f1. I create an empyt 2D project, and import a asset like this:
or this

After importing I save and test the demo scene (playing the level in the editor), the framerate is stable at 120-140fps

Then I close the project and Unity3d. After loading the project, I test again the demo scene and then the framerate drops from 140fps to 60-70fps every 5-6 second.

Both assets have the same fps-drop, on a 2D scene and at a 3D scene too. I don’t know why, is this a bug from the editor? I’m new on the unity engine I have no experiene with older version of unity.