This is going to seem awfully rambly, I’ll try to keep it short.
I have identified that reading text upside down is more difficult. The scale goes like this: baseline is reading normally, then there’s reading letters and words that are upside down, then letters and words that are mirrored, and finally, although I haven’t tested it recently, letters and words that are both mirrored and upside down. This is obviously a scale that goes from easiest to harder. Not hardest, as I’m sure there’s more complex variants (applying a random tranformation out of those four to every single letter and then again to every single word comes to mind as a simple but really-hard-to-read variant).
Say you are wearing a t-shirt with some text on it. To an outside observer, the text is upright, difficulty level 0. Now look down at your own t-shirt: is the text upright or upside down? I.e. is reading it level 0 or 1? Now take the bottom hem of your t-shirt and raise it, up towards your face, so that the fabric is now roughly at the same level as your face, and try again. Note the results.
For me, in the first instance, the text looked upright, level 0; in the second instance, the text looked upside down, level 1.
However, if I look at this from outside my own experience, and think of my eyes as cameras, the text was upside down all the time. NEVERMIND I FIGURED IT OUT I AM STUPID.