The Greeks ignored word boundaries in their quantitative verse, though this is hard to wrap my head around...Latin only did occasionally--forms of elision between words that ended with a vowel (or a vowel plus a nasal) when the next word begins with a vowel--which, by the way, is why poets in the Renaissance (& later) would copy that with (to our reckoning) th' use of ill-advised apostrophes...
The main issue, it seems to me, is that modern English has a lot of short-vowel nouns which frequently enoughoccur as stressed, but which would be counted by the old system as dips. (These words often would have had inflections in their earlier form.) now, i think it is fairly obvious that sh, th, ng are single sounds, while ch,j, & x are double consonant sounds (t-sh, d-zh, k-s or g-z)--per my essay. What i didn't get to, there (& which ultimately caused me to abandon quantitative writing) is two very inconvenient facts:
speed of utterance (& even length of line) changes vowel quantity.
"Loving her was easier than anything i'll ever do again" (i like Kristofferson because he wasn't afraid to use long meters).
This, of course, scans as a perfect iambic tetrameter line, followed by an iambic pentameter. But when he sings it, you recognize that it's actually double-time, with beats on "loving", "easier", "anything", "ever" & "again". I won't say the stress on the first syllable of "loving" is enough to turn that into a long-quantity, but in that line it's distinctly longer than the syllable (with identical vowel) "was"...
I read some of my drottkvaett poems out loud, & discovered a distressing tendency to make them anapestic doggerel, just because, in my mind composing, i was saying them at about half my natural speaking rate, which throws smaller stresses into higher relief.
I rather doubt there was any such thing as silent, speed-reading for Old English poetry.
the clash between successive consonants affects vowel quantity.
We don't even have a name for this! (I called it "dit'tink".) But English (as well as Germanic & Slavic languages--& unlike Romance ones) creates this effect all the time. "Fact checking" for example. You have to stop your breath completely in order to say the sound clearly at the beginning of the next word. (What happens in speaking, of course, is that the voice at the same time changes pitch.) The vowels in "top" in "top notch" & in "top tier" are different--longerish in the second, in effect--because it takes more effort to distinguish the two sounds. Examples can be multiplied ad infinitum.
Again, this is something that happened when our words lost most of their inflected-endings.
This doesn't mean that vowel quantity doesn't exist in English--nor that it can't, at some level, be subjected to what i called "disemic sorting"--but it does mean that such sorting is based on a deliberate simplification of the actual sounds; one that happened once for Old English & which created few difficulties (&, note well, they also had rules about which parts of speech could fill those same phonemic functions--this, i think, made figuring easier for them); i think that writing in an adaptation of alliterative meters will probably follow a natural progression--from at first equating a stressed syllable (as we are used to perceiving) with the basis for a lift, & then perhaps later being able, in composition (& in hearing?) to find that there are syllables that can be improved by paying attention to vowel length. but the important thing is to train the ear for the overall rhythm, the beats & the pauses--something that our pop music (which is still almost invariably in 4:4 time) has never forgotten.
somehow the link to the original essay didn't copy. here it is:
http://www.pearltrees.com/graywyvern/poetics/id9812919#item95337050