You can enable low-processor mode to reduce CPU & GPU usage. See related FAQ https://docs.godotengine.org/en/stable/about/faq.html#is-it-...
func _ready() -> void:
set_physics_process(false)
PhysicsServer2D.set_active(false)
PhysicsServer3D.set_active(false)
Engine.max_fps = 1
RenderingServer.render_loop_enabled = false
get_tree().paused = true
I wonder what's using that CPU?