T-Pain has either attention deficit disorder; something that prevents him from performing songs in their entirety; or the inability to reproduce his hits outside of a recording studio.

Or, perhaps he suffers from a combination of the three.

When the 25-year-old rapper performed at Rehab on Sunday he did more talking than rapping. He did more dancing than rapping, too. And when he did rap, he did so over recorded tracks – and never for long.

The performance could be described as a lot of standing around and talking with short, single verse-long bursts of T-Pain karaoke in between.

The bizarre showing of sampled songs certainly wasn’t due to T-Pain’s lack of material: He and Kanye West have a Grammy Award-winning hit, “Good Life,” and he collaborated with a handful of other big names on more than a handful of other big singles, including with Jamie Foxx on “Blame It”; with Flo Rida on “Low”; and with Chris Brown on “Kiss Kiss.”

T-Pain has a few hits of his own, as well, and three albums (a fourth is on the way) to draw from.

What’s more, earlier this year the rapper appeared in a Saturday Night Live digital short that became a YouTube sensation. The song, “I’m on a Boat,” was so popular that it was remixed into a club track.

T-Pain performed part of the SNL single, along with snippits of “Good Life,” “Blame It,” “Low,” and “Kiss Kiss” Sunday afternoon – but none in their entirety, and none were truly performed live.

Instead, T-Pain sang a verse and/or chorus of each song on top of the studio version. He made no effort to hide his lip-synching either, and would routinely let the mic drop to his side as he joined the choreographed routines.

His so-called “band” consisted of a DJ at a laptop in the back, a male backing vocalist/dancer/crowd-enticer, and two rather eccentric and energetic female back-up dancers.

The Asian dancing duo were reminiscent of Gogol Bordello’s Pamela Racine and Elizabeth Sun – except Racine and Sun mix some percussion into their entertaining dance routines.

The show started at 4:30 p.m. and the bathing suit-clad crowd at the Hard Rock pool was well-watered and in good spirits.

While the revelers seemed to enjoy the first few samples, many soon grew tired of the long pauses between songs, and onstage banter that was virtually indecipherable.

More were lost when songs were cut short time and time again to make way for more inaudible chitchat and nonsense. It was the musical equivalent of an hour-long cock tease that left the rap-rejected crowd with hip-hop blue balls.

The increasingly irritated audience soon thinned as revelers decided to keep the party going – without long pauses between bursts of Labor Day Weekend fun – elsewhere.

Meanwhile, a few of those who stuck around began voicing their displeasure.

“Tell T-Pain to shut the (expletive) up and sing!” one woman told the muscular man who stood next to her.

T-Pain did not get the hint.

A handful of disapproving shouts declaring, “T-Pain you suck!” were scattered throughout the performance.

The crowd weren’t the only ones hating: At least twice during his 60-minute set, T-Pain stated, “Jay-Z sucks.”

Unfortunately, it was unclear why he felt that way – or the need to say so, repeatedly, during what was supposed to be a fun concert at one of the biggest Labor Day parties in Vegas. It was virtually impossible to make out what T-Pain said before and after his unsavory proclamation.

There were, however, a few high points yesterday at the Hard Rock. The first came during an unusually quiet part of T-Pain’s performance, when the rapper was actually audible between songs.

He told the crowd that his new album will be called Revolver and noted how, if you remove both occurrences of the letter “R” from the word, you get “evolve.”

He shared how he found it amazing that something so inspiring as the word “evolve” could come from “something so violent.”

It should be noted, however, that this contradicts reports that the rapper’s forthcoming album will be called Uber.

T-Pain, whose real name is Faheem Rasheed Najm, then proceeded to perform a song (or the better part of it) from the soon-to-be-released record. It was at this point that he appeared to abandon his Ashley Simpson-esque antics of lip-synching and silly dance moves.

Though his set was hardly high-energy or fast-paced, the rapper further slowed things down about 45 minutes into the performance when he took a seat at a keyboard at the side of the stage.

While he gave what could have been a relatively moving solo performance, poor sound levels continued to plague the poolside concert, and it was hard to hear what was coming from the stage. As a result, the misguided ballad was lost amongst what was still a very party-hearty crowd.

Yet, if the rapper noticed the crowd’s disinterest, he didn’t let on. He performed, without his signature top hat or sunglasses, for exactly an hour.

Among the thousands of partiers at Rehab yesterday was rapper, Fabolous, and songstress Rihanna, who looked on from a VIP cabana above the stage.

The stunning star seemed to enjoy the show, despite what was likely a somewhat awkward moment, when T-Pain performed a shortened rendition of his collaboration with Rihanna’s ex, Chris Brown.

The former couple made headlines earlier this year after photos of Rihanna’s bruised and beaten face surfaced following a violent and highly-publicized domestic dispute.

Rihanna’s face was brave, unbruised and beautiful yesterday, however, and she was all smiles, despite the brief reminder of Brown.

Meanwhile, some fans seemed to enjoy what was going on above the stage more than the performance onstage. While many snapped pictures and shouted and waved at Rihanna, one disapproving member of the audience effectively summarized the general mood and loudly urged T-Pain to “get off the stage and let Rihanna sing.”

Unfortunately, the suggestion, like the rapper’s inter-song banter and authentic material, fell flat, upon far too few ears.