Is Timer elapsed time consistency depends on resources?

On a small project (10 sprites static) i use one scene timer. When monitor the elapsed time i got consistent seconds passed…
But after adding one more sprite (just an image without events, but with an effect = Drop Shadow with Quality=20) the timer goes slower… and when monitor the elapsed time i got inconsistent (lower than real) seconds…

  1. when add one more instance of the above sprite the timer gets even slower…
  2. when change the Quality=1 of the Drop Shadow effect, the timer gets NORMAL!

Any explanation?

found that for a solution: