There are two sides to Gilbert Kaplan's argument. The first is a simple matter of clock time. Over the years, he says, Adagietto has relaxed into Adagio, and latterly into Adagio molto. Supporting this claim is much easier than in the case of 'period' Beethoven tempos: Kaplan has timings in plenty, from Mahler himself to the present day, and one or two significant recordings to prove it. Mahler's own performances, and those of dose collaborators such as Mengelberg and Walter clock in at about eight minutes. Today you find performances that stretch it out to quarter of an hour.
As far as that goes, it's quite correct. The trouble with timings, however, is what they don't tell you. Would a performance of, say, ten minutes be generally slower in tempo than Mahler's—or could it be the same basic pulse, but with a lot more rubato? And then there's articulation. How you point a phrase can make a huge difference to what you might call its 'psychological' tempo. If that sounds disgustingly airy-fairy, let me give you a concrete example. Recently I heard two recordings of Schubert's Sixth Symphony. In the first the pace in the Adagio introduction struck me as pretty sluggish. For comparison I turned to recording No. 2: much livelier, I thought, until I got the metronome out. Then came the revelation—the beat was almost exactly the same. What made the difference was the phrasing—sharp and purposeful in recording No. 2, limp in No. 1. In Bruno Walter's 1947 recording of Mahler's Fifth Symphony (Sony (D CD47683) the Adagietto is slightly shorter than in the Kaplan—7'40''—but to me it feels more expansive. The way the performance phrases is quite different.
I'm less sure still about Kaplan's second point—that the restoration of the authentic timing converts this from something ''funereal'' into ''a simple expression of love''. Surely one doesn't have to be a Freudian to see how intense feelings of love and awareness of death can go hand in hand—especially in the case of a complicated soul like Mahler. Even if you accept Kaplan's point about basic tempo, that still leaves plenty of scope for the interpreter.
Even so, I think that in essence there is a valuable lesson to be learned here. I agree that performances of the Adagietto have been getting ridiculously slow of late. Some performers—like Bernstein in his DG recording (2/92)—can carry it off movingly at 11 to 12 minutes. But unless you have Bernstein's mastery of slow phrasing plus a superbly disciplined string section—you can end up losing the sense of line altogether, as in parts of the recent Wit/Naxos version (see above). In some passages it's almost like being cast adrift in a sea of sound—very nice, but quite shapeless.
As for Kaplan's performance, well perhaps it's because I read the notes first, but I can't help sensing a didactic impulse behind it: most of the time it's too careful to suggest consuming passion. Still, as a demonstration of the virtues of keeping the movement fliessend (''flowing''—the marking appears three times in this movement, plus one Nicht schleppend, ''not dragging'') it has force. Hear it and ponder, and then get the 1947 Walter complete recording of the Fifth Symphony to see what an unstretched Adagietto can mean in context.'