I have some information that may interest all who have experienced this issue.
I’ve been testing out this thing quite a lot lately because, while when the thread was started I made a quick try and observed no delay or particular issue, I have since then, on occasions, noticed how unresponsive the sensor can indeed be.
I don’t know how the sensor works, but I suppose it is a light sensor of some kind, probably the same used by the automatic brightness utility.
Supposing my understanding is correct, I used my finger to block light from it while calling (I give the the sensor is that translucent thingy that can be seen at the opposite side of the front camera).
The switching on and off of the screen is consistently instantaneous with the finger, but when I take the phone to my ear, not so much.
What is the difference?
Well, not the angle of the phone, because I tried to block the sensor with the finger while standing, while lying on bed and in pretty much any position, and it always worked.
I though that stunning or feeble lights in the surroundings could be the culprit for the inconsistent behavior but my tests with the finger hovering the sensor always produced successful results regardless of whether I was in broad daylight or pitch dark, under the light of powerful lamp or a candle.
Then one day when the call was getting too long and I wanted to switch to the headset for comfort, I had to brush my hair away from my ear (I’m long overdue for a good grooming! ) to put the earplug in, and while I was doing so, from the corner of my eye I saw the screen turn off instantly as the ear was free from my hair and it occurred to me the hair could be the problem.
I have tried then all sorts of positions and when I have hair between the phone and the ear, the sensor is faulty.
My hair right now is just about as long as the lower part of the ear, so depending on the wind or how I scratch my head, the ear may be anything from entirely covered to completely free from hair.
I suppose the general fuzziness around the ear is confusing the sensor.
So, in conclusion, to all those who have experienced this issue: how is your hair?
Seriously though, can you try to experiment with the finger over the sensor in different light conditions and then repeat with or without hair over the ear, hats, hoodies and whatnot?