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.
Only about half the glyphs to recognize. Being able to skip further down the "page" and trivially continue reading. Being able to rapidly scan a line of text.
I guess this is all predicated on a mostly literate populace with a lot of reading material. You probably don't need to rapidly scan a wall carving extolling the virtues of your God-Emperor and recording all of his battle victories, unless you're trying to win a bar-bet. ("I'm telling you, he beheaded TWELVE infidels with a single swing!" "Yeah, well, find it on the wall and I'll buy you a beer.")
This is not really a big deal. Some writing systems require you to recognize a few thousand symbols, and I read (i.e. "effortlessly recognise") three alphabets and with some concentration two more (the grammar is a bigger deal for the latter...my overall fluency in those languages is, at best, poor and I am sure will never get better. But symbol recognition is not the barrier).
This is because I put in the effort to study these languages, but many people, without much formal education, can read that number of alphabets (much better than I can) simply because they live in (typically border) regions where that is reality of daily life. My own mother grew up in a town where four writing systems were in use, plus used a different alphabet in her home.
Don't mock that older writing by the way. Most people were illiterate; Especially anything in paper was the writing of some pretty sophisticated folks.
1 - The earliest ink-and-paper writing in the Middle East was in Egypt, where the pen writing mostly followed inscription, being a little free form in ordering. AFAIK the old language didn't acquire an alphabetic system until Greek colonization (cf Coptic, which is (evolved) pharaonic Egyptian written with essentially Greek characters). That latter is indeed a L-R system, but has been largely replaced with the R-L Arabic.
2 - Your hand is likely to smudge R-L systems unless you're left handed which most people are not.
3 - The other in-and-paper system, Chinese, until recently (couple of hundred years ago) used only brushes, not pens, so smudging was a non-issue. Top-bottom though could be attested in this way; it's better to draw the brush rather than push it, so starting at the top of the page makes sense, but why R-L vs L-R...?
I don't know about the the third independent site of the development of writing, the Americas, as the jesuits burned almost 100% of the books, and so mostly we only have monumental writing, which doesn't address this question.
AFAIK all the indigenous Alphabets of India are L-R (Urdu is R-L due to its use of an arabic alphabet)
Arabic was originally read Right-to-Left, but written Top-to-Bottom, leftwards, on the scroll so smudging by right-handed writers wasn't an issue. A completed scroll was then hung sideways on the wall for people to read. In fact, by writing Arabic T-B(leftwards) instead of L-R(downwards in columns) like Greek was at the time, smudging became less of an issue. The Arabs could roll up the left side of their scroll sooner without smudging or pausing, whereas the Greeks had to leave the entire column they'd just written exposed while the ink on the bottom rows was drying.
As it happens the big-endian notation of hindu (aka "arabic" in English) numbers is kind of a botch in L-R languages since you have to parse the entire goddamned thing to learn anything about it (and when it's long you have to parse it r-l anyway y divided sections). At least in R-L writing you can get evenness immediately.
No idea what the relative cost of those are but I'd wager it exceeds the mechanical cost of scanning back to the "front" of the line...
So it's definitely not twice as much work, I'd say it's not more work at all, you basically just don't worry about the chirality and that's it.
This is only really the case for phonetic writing where you learn to recognise the shape of groups of letters (this is how you are reading this text). For logographic/pictographic writing it is much less of a benefit.
And as I wrote above, even for alphabetic writing like Greek it doesn't really matter if all you read is the occasional monumental inscription.
Take Japanese writing for example, you definitely don't parse individual kana. Same with hangul, as you mention.
Even for Chinese writing it is true but to a very small extent, but my point was that it is much more important for phonetic writing, and most important indeed for alphabetic. I have even heard that it is less pronounced in Cyrillic, because all letters basically look like majuscles so the outline of words is quite uniform.
I don't quite get the comment about alphabetic script not being very phonetic, it's true that this would promote parsing words as units, but the main reason we do it is because it's much faster, not because some spellings are opaque. Kana spelling is almost completely one-to-one but it still makes sense to read e.g. -ma.shi.ta as a unit.
Inventing writing is really, really, really hard. Just as, given that we learned to read so long ago, it's hard to remember how hard it is to learn to read. Boustephedron is like training wheels for writing.
And since writing on stone is a pretty cumbersome activity it was probably natural to continue writing next to where you just finished a symbol.
Note that cuneiform (which of course predates Greek by a wide margin), written on clay with a reed, was not written boustrophedon.
On the other hand, Egyptian hieroglyphics, though carved in stone, weren't either - instead they were very flexible on writing direction.
I suppose my only real point is that the Ancients didn't see a great need for a fixed writing direction.