Ok, lets test in the same way and see what happens.
I ran Antutu Stress Test at full brightness with the following results. 88% 2m 27s, 87% 2m 02s, 86% 2m 07s, 85% 2m 1s
I don't know why the first % lasted a little longer. Maybe the stress test was less intense during that % but you can see it's pretty stable.