I also could tell that a couple of editors didn't finish my MS. One actually wrote "I read deep into the MS" which explicitly indicated they didn't finish it. Another was really complimentary of the writing but put, as an example of what she loved about the writing, a passage from like page 7. Which made me think she didn't read much farther than that! In fact, other than a few who commented on the ending and the one who said they took it to an editorial meeting and had other editors read it, I wondered if any of the rest had actually finished the book. One mentioned something about substance abuse, which I did not consider to even exist in my MS, and then I realized that in the first 10 pages the MC is struggling to get off Xanax after taking it to cope with a death-- but after they do, it never comes up again; that too made me think they hadn't gotten very far, since substance abuse is not part of the plot. So yeah, I'd guess some of the editors who passed on yours probably did not read past page 50. Though to be fair, I read for a poetry book award, and if the first couple of poems don't grab me I usually just move on. So I guess I can't blame them.
It would be nice if there were some kind of standardized form they had to fill out indicating how far they read and specific questions about plot, characters, pacing, etc. so that we could derive something from the passes. But I guess their jobs are hard enough already.