What was the reason for writing this way on stone? It can't be to keep text aligned - you can just walk to the left and keep going, and the text will stay aligned just as well as if you'd been writing right-to-left.
I don't ask this as a joke: is there seriously a cognitive advantage to having to pass your eye all the way back to the beginning of the next line (whether you read right-left, left-right or top-bottom) vs immediately dropping down and continue reading?
On the other hand I spent a lot of time in high school and university reading pictures of boustrophedon greek texts and once you are used to it you don't even think of it. Likewise R-L texts -- my difficulty with handwritten arabic is the wide variation in characters; the directionality rapidly became automatic, even in mixed-language texts.
In the case of boustrophedon texts, a dominating factor is the sheer irregularity of character shapes (nothing was machine made) and worse, a lot of the black and white photos in books are old, crummy ones which make the deciphering even more "interesting". And this was all before the development of spaces between the words too, and at a time when character shapes were evolving.
Regardless, all written languages I know of (even counting Korean, which is slightly different in-word), seem to have picked a single direction as a dominant, and even before the development of mechanised writing (which would make the characters extremely regular, which might drive a tend to optimize). So there could be some cognitive benefit. I could imagine there's an advantage in that you could train a NN in your visual system to recognize regularly-shaped characters and do look-ahead, but I have seen no studies of it.