On YouTube, I change the speed constantly, and, sometimes, I think I’m watch at x1 when I’m actually watching at x1.5 or even x2.
I can usually differentiate x1 from other speeds, but when there’s a slow talker and no music, it’s actually quite easy not to be able to tell unless I think hard about it, or realistically just check, especially since I’m very used to it, having done that for around two years.
However, watching a show for 30 20 minutes without realizing it is unrealistic, especially since there are a lot of sound effects and it’s 30 20 minutes!