"...the faster they can play, surely the more masterly they must be..."

"The notes I handle no better than many pianists.
But the pauses between the notes --
ah, that is where the art resides!"

Pianist Artur Schnabel wasn't really a folk singer, but I think he adds to two viewpoints in this thread.

The first, more musical, is that it's not just the tempo (and not just the notes, not just the lyrics, not just the instrument) -- it's all the pieces in relation to one another. Haven't you ever heard an old favorite sung by someone other than your favorite singer, and had an epiphany from an unexpected change? I've known the Gaelic song "Ho Ro Mo Nighean Donn Bhóidheach" since childhood, and in Cape Breton it's usually uptempo. The Rankins recorded a much slower version, coloring the song with poignancy.

Schnabel's other point, and not the one I think he had in mind, is that some performers get very artsy. They've fallen in love with the song, or with the tune, or with their own voice, or with their own playing, and get carried away when perhaps they should be carried out.