In metrical poetry, what is equal timed (isochronous)? Is it:
- the foot, or
- the ictus, stressed syllable, in a foot, or
- something else, or
- nothing in particular?
If anything is equal timed, it's the foot.
You can see that it's the feet are approximately equal timed by listening to poetry that has both iambs and anapests for feet. The anapests are three syllables and the iambs are two syllables, but the syllables in the anapests are generally read more quickly than in the iambs, so as to give each foot roughly an equal length. Poetry like this (sometimes called dolnik meter, from a Russian term) didn't appear in formal poetry until the 19th century, when Samuel Taylor Coleridge started using it, but it had long been used in nursery rhymes and folk songs.
One example of this meter is Thomas Hardy's poem “Neutral Tones”. You can hear it read on YouTube here. The first three lines (scansion added by me) go:
x / x x / x / x /
We stood | by a pond | that win- | ter day,
x x / x / x x / x x /
And the sun | was white, | as though chid- | den of God,
x x / / / x x / x /
And a few | leaves lay | on the star- | ving sod;
You can see that some feet have two syllables (these are iambs), and some have three (these are anapests). If you listen, the syllables are pronounced more quickly in the anapests, and this tends to make the feet equal lengths.
You can also hear that the feet aren't completely equally spaced. The one spondee (the foot leaves lay, which has two stressed syllables) is longer than the iambs. And the speaker pauses at the ends of clauses, thus adding a pause between some feet.
We can compare poetry to music in this sense. In music the notes in some sense start out being equally spaced. But often, there's syncopation, which makes some notes get delayed or rushed; some notes have fermatas, which mean they are held longer than they would otherwise be, and there are accelerandos and rallentandos, which means some sections are played faster than others. So the metronome beat of equally spaced notes is merely a starting point, and a performer or composer can vary from it. But in some sense, the beat is still there and serves to organize the music, even though the music departs from it. This is essentially what feet do in poetry.