“Accelerate” is what its title suggests: a return to speed for R.E.M., the godfathers of alternative rock. Given the lukewarm responses to their last three decidedly-subdued albums, the band should be commended for cranking up the volume, letting loose, and creating their shortest album ever. But the results, though promising, aren’t immune to R.E.M.’s recent strains of mediocrity.

The single greatest aspect of this album is the triumphant return of Mike Mills’ musicianship. As R.E.M.’s trusty bassist, Mills has been criminally overlooked for decades. Household name Michael Stipe has always been known as a bit of an enigma, Peter Buck has his signature ringing guitar, and Bill Berry’s got his oft-lamented departure from R.E.M. to become a farmer.

But Mills is R.E.M.’s quiet hero. His killer basslines made “Murmur” a masterpiece; his high-pitched vocal harmonies—the perfect counterpart to Stipe’s low mumble—propelled songs like “It’s the End of the World as We Know It” to holy levels of catchiness (that song wouldn’t be half as good without the “time I had some time alone” backing refrain).

After 12 long years hidden beneath unnecessary over-production and forgettable songs, Mills’ vocals burst to the forefront and enliven much of “Accelerate.” Whether channeling the Beach Boys (“Man-Sized Wreath”) or Neil Young’s Crazy Horse (“Supernatural Superserious”), Mills’ sing-a-long harmonies, present on nearly every track, make the solid songs great and the mediocre songs good.

Stipe, too, sounds reinvigorated, the most energetic he’s been since “Automatic for the People.” On standout tracks like “Living Well’s the Best Revenge” and “Mr. Richards,” his delivery calls to mind the vitality of R.E.M.’s late-’80s albums like “Life’s Rich Pageant.” The melodies have a perfect mix of ringing and urgency, the aforementioned harmonies soar to heavenly levels, and the songs rock from start to finish.

The best track, “Supernatural Superserious,” sounds like a lost ’90s classic. Despite its dopey title, the verses’ chiming guitar refrain and the infectious chorus make it the catchiest, most ecstatic tune R.E.M. has produced in the last decade.

The album’s primary challenge is overcoming a sense of incompleteness. Many of the songs midway through the record suffer from a strong chorus with weak verses, or vice versa.

“Hollow Man,” for instance, initially sounds like the pedestrian piano-driven ballads R.E.M. has produced all too often the last few years. But after a minute, the chorus explodes with drums and Mills’ chanting, and the track vastly improves. “Until the Day is Done” starts off with boring, war-related lyrics and distant drums but eventually gathers momentum to a pleasant, if generic, climax. And the verses of “Sing for the Submarine” sound sinister and vaguely creepy, but give way to a pretty, more complex chorus.

The brevity of the album also has important consequences. Some of the faster songs end strangely and abruptly—in an unnecessary whir of airplane propellers (“Houston”) or guitar shredding (“Accelerate”)—and the effect is off-putting. When they hit the right notes, however, the succinctness of the songs demonstrates a maturity. Tracks like “Horse to Water” are perfect punk gems, sounding kind of pissed but joyful and anthemic at the same time. In that sense, “Accelerate” is like a better version of 1996’s “New Adventures in Hi-Fi,” which had a similar rocking feel (and perhaps even superior songs, as a whole), but took a whopping hour to slog through.

The two-minute nugget “I’m Gonna DJ,” which R.E.M. played heavily on their 2004 tour, is the perfect closer to the album. Though they wrote the song before their last release “Around the Sun,” it’s easy to see why it wasn’t included on that album. “Accelerate” is everything “Around the Sun” wasn’t—short, fast, noisy, urgent—and the title track embodies all of this. Mills throws in “woo-hoo” harmonies and Stipe spits one-liners like “Death is pretty final / I’m collecting vinyl.” The song is a reminder of why R.E.M. became popular in the first place.

Like all bands that demonstrate a certain degree of longevity, the question remains whether to judge R.E.M. by previous standards of greatness or by their recent output. The album clearly doesn’t measure up to the I.R.S. classics, “Out of Time,” or “Automatic.” But in their post-Bill Berry careers, “Accelerate” is a landmark, because it proves that R.E.M. still remembers how to rock. “It’s a new day today and the coffee is strong,” Stipe declares. “Accelerate” may fall short of R.E.M.’s earlier glory, but at least the band has some pep in its step.