UPDATE! I found more about the bug/glitch. It's linked to pausing!
That thing took me on a weird, wild ride... At first, I thought it was linked to how LONG you stayed paused (don't start looking into it right away, that's not actually it; I was mistaken). I paused, waited, looked at the time, unpaused, and there it was. I paused, quickly unpaused straight away, and it was gone.
So my theory was that it was linked to how long was your latest pause. The shortest time it took for the glitch to happen was 15 seconds ("wow, that short!??", I exclaimed). So if your latest pause was > 15 seconds, looking around would not slow down time. If it was shorter than that (or if you had not paused since entering the game), it would.
So, I tried to set up a recording of this. Set up a chronometer hooked to the "Escape" key, and used a program to record my screen, with the chronometer in sight.
I pressed "z" to un-pause (I was paused, and pressing escape would have started the chronometer; I wanted it to count when I was paused, not the other way around), and used C to look around. The game wasn't slowing down. Unsurprising, I thought, since it had been paused for a while now.
So then I pressed "escape" to pause, and waited 5 seconds. pressed "escape" again to unpause (also pausing the chronometer in the same keystroke, now displaying 5 seconds), and used C to look around. Time was slowing down as expected.
I continued, this time waiting 10 seconds. Looking around, time was slowed down. Nothing special here, only building up to when it WOULD slow down.
Again, this time waiting the dreaded 15 seconds. Looking around, time was... slowed down?? O- okay? Maybe I was wrong and the trigger was something like 18 seconds? Let me try again, this time waiting 20...
So I did, waiting 20 seconds... Looked around using C... time was slowed down. Huh???
Did it again, this time 30 seconds. It was STILL correctly slowed down.
At this point, many questions were running in my head. My first thought was: is either the recording software of the chronometer preventing the glitch from happening?
So I closed both, waited I-could-no-longer-be-sure-how-long seconds, and unpaused...
The time
was NOT
slowing
down.
I was now about to rage about the time I wasted setting all of this up since it was now useless, but it barely took me about 2 seconds to realize what I had just done...
I used Z...
to unpause...
Dumbfounded, I tried it again. Pressed Escape (pausing), then Z (un-pausing by selecting "RESUME")...
Looked around with C...
The time was NOT slowed down.
This glitch is not triggered by how LONG you last paused, but by HOW you last UN-PAUSED
Un-pausing with Z (i.e. "selecting RESUME") triggers this glitch
Un-pausing with Escape makes things go back to normal
The only reason I came to my previous conclusion was because when I would want to quickly pause-unpause, I would double-tap Escape
However, when I left the game paused for a while, I would obviously put my hands away from the keyboard, and once came the time to unpause, hitting "Z" would always be easier than having to reach for the "escape" key again...
... I feel dumb :|