Doesn't matter the speed of flickering, I am affected by PWM even on the new MacBook Pros, which flicker in the 23,000Hz range (or something like that).
You don't 'see' it, you don't even feel it instantly, you just start to feel like there's something wrong with your eyes after a half hour or so, and it seems to get worse on repeat exposures, so that eventually you DO 'see' it as soon as you look at the screen -- or more 'feel' it as 'woah I can't look at this, this hurts my eyes'.
I had to return my iPhone X for the same reason (which i didn't even realize had PWM until i found after a couple of weeks that i just couldn't look at it any more, and subsequently discovered all OLED has PWM), and am back on a non-OLED iPhone 8 now.
If PWM doens't affect you, lucky you. But it's real at any speed for those of us it does affect.