I don't know if a 45-minute show qualifies as the perfect length or a complete rip-off, but I know 45 minutes in Sleigh Bells' hands is time well spent.---

The energy of the sold-out Urban Lounge was palpable well before the Brooklyn duo took the stage in front of their small wall of Marshall amps, with the house sound system pumping a non-stop batch of worthy dance tunes: Beastie Boys, Jay-Z, Kanye West, some Motown.

Then, with the inexplicable aural appearance of a sample of Fleetwood Mac's "Seven Wonders," singer Alexis Krauss and guitarist/programmer Derek Miller took the stage and promptly delivered a rapid-fire run through most of their debut album, Treats, taking the energy in the place to a whole new level.

It was hard to tell how much of Sleigh Bells industrial skronk-meets-hip-hop was actually being performed live, and how much was preprogrammed, but in the moment, it didn't matter a bit. Krauss prowled the stage, delivering her cheerleader chants and off-kilter raps in songs like "Tell 'Em," "Kids" and "Infinity Guitars" with style and, again, manic energy. She was a blast to watch, all flying hair and head-banging dance moves. Miller, on the other hand, was a bit of a mystery. He was playing guitar some, but  disappeared completely from the stage at times.

It didn't matter, because Krauss is clearly the selling point on Sleigh Bells, and she proved to be quite the charmer. Besides chatting up the crowd between songs and leading a few singalongs/clapalongs, she delivered one of the most unusual encores I've ever seen: An apology that the band was all out of songs to play! The duo had built in an encore about a half-hour into the show, but even when that was done, the people weren't going anywhere, chanting for more.

Fresh out of material, Krauss returned to the stage to thank the crowd once again, and to beg the house DJ to start playing some music so the dead air wouldn't simply be filled by the fans' disappointment that the Sleigh Bells dance party was done less than an hour after it started.