When SpotLight is turned on the first time, rendering blocks for around 1000ms. Consecutive toggling of the visible property is smooth and doesn't cause performance degradation.

Example: Dynamically turn on SpotLight
  Given Quick3D SpotLight is in the scene
  When spotLight.visible property is toggled for the first time
  Then I should immediately see the SpotLight
  But rendering blocks for approximately 1000ms 

Example code is available in the following block. vehicle.leftBlinkerOn is exposed from the C++ side but can be also simulated using a Timer.

SpotLight
{
    id: spotLight
    visible: vehicle.leftBlinkerOn
    color: Qt.rgba(0.96, 0.55, 0.0, 1.0)
    brightness: 25.0
    position: Qt.vector3d(-1.0, 0.7, 5.0)
    rotation: Quaternion.fromEulerAngles(-20.0, 175.0, 0.0)
    constantFade: 1.0
    linearFade: 0.0
    quadraticFade: 1.0
    coneAngle: 35.0
    innerConeAngle: 25.0
}