Katy Perry -- Britney Spears Concert ... I'd Rather Watch Paint Dry

Britney Spears blew everyone away during her Vegas debut last night ... everyone, but Katy Perry.

Katy was front and center for the "Piece of Me" show -- rubbing elbows with Miley Cyrus and Adam Lambert -- and while Miley danced her ass off, Katy couldn't seem less interested.

True enough ... Britney's no John Mayer, but come on Katy -- she's a much better dancer than him.

As they say ... you can please some of the people all of the time ...