Now I have started working on my next build for the S7 and came across with this graphics performance improvements in the hadk-faq.
I added that extra line (QPA_HWC_IDLE_TIME=5) into the '/var/lib/environment/compositor/droid-hal-device.conf' and it indeed reduced the red lines in my frame rate test. (There were not so many before but I got now even less.)
So, I recommend this little extra manual work to you as well.

Note: I have tried the other QPA_HWC_BUFFER_COUNT=3 line too, but it made the frame rate test a bit worse, so I removed this and only kept the idle_time one.