The piece is a single, continuous river of fragments. Brief motives, built-up clusters, swaying-pendulum harmonies are repeated but also circled, considered; rhythms and emphases shift, the modules contract and expand. An eight-note melody sporadically returns, mantra-like. Sharper sounds periodically reset the machine: a funky burst of cello pizzicato, a clutch of knife chords. The second half coalesces into longer passages, much of it settling into a lean, glacially luscious chorale. Amid the occasional callback, the last hour spins variations on isolated pairs of notes. But by then, one’s ear — and clock — have been thoroughly recalibrated.

The Flux Quartet (violinists Tom Chiu and Conrad Harris, violist Max Mandel, and cellist Felix Fan) are the current, undisputed champions of Feldman’s Second, having given the first full performance of the piece in 1999. (The 1984 premiere by the Kronos Quartet was somewhat abridged.) Friday’s reading was a touch more brisk than usual (other performances have regularly topped six hours), but was still governed by insistent calm. Their strategy for meeting the work’s challenge — absolute physical efficiency, nary a wasted movement — generated a meditative aura of internalized skill, jewel-like sounds emerging from the workshop with low-key grace.

Feldman sometimes described his working habits as a respiration between fancy and craft: As he drafted each page of a score, he said, he would immediately, meticulously copy it in ink, all the while thinking about what should come next. That open-ended, deliberately industrious rhythm pervades the Second Quartet, epitomizing the implications of Feldman’s modus operandi: The music seems to take on the exact pace and dimension of the compositional process. The extreme length ends up solemnizing the spark at the heart of that process: The players’ concentration and stamina fostered, in essence, an all-night vigil over the mysteries of creation. Painstakingly, methodically transcendent, Feldman’s Second Quartet is, in a profound sense, all in a day’s work.