of Tempo and Early-Music Performance:
by Bernard D. Sherman
Early Music, August 2000
Bernard D. Sherman is the author of Inside early music (Oxford University Press, 1997), co-editor of Performing Brahms (Cambridge University Press, forthcoming), and author of the essay 'Authenticity in musical performance' in The Encyclopedia of aesthetics (OUP, 1998). His essays and articles have appeared in Early Music, The New York Times, and many other publications. His Web site is www.bsherman.org
AUTHOR'S NOTE, June 2014: I've decided to take this one down for repair (or perma-retirement) - I've really changed my views on this topic since this article was published, and while I can't unpublish it, I can at least stop propagating it! Besides, there's something WAY better for free online now - Ido Abravaya has posted his superb 2006 book On Bach's Rhythm and Tempo as a pdf. Go though and read it at the link! A must for everyone serious about Bach interpretation.
Another helpful statement comes from John Butt discussing his own 2010 recording of the Mass in B minor:
"....Don Franklin has suggested in a study of the Missa of 1733 that Bach's system of tempo relationship might share something with the proportional system of the Renaissance era, although differing from this in significant ways. According to Bach's pupil, Johann Philipp Kirnberger, tempo should be based primarily on the choice of time signature and the notational values used. Each signature relates to a normal tempo' (tempo-giusto) as held by its principal beat, and this is modified by the predominance of shorter or longer divisions (with more shorter divisions it would thus be slower, with longer notes it would be faster). This rule of thumb is then further inflected by Italian words, as necessary, which modify what might have been expected from the time signature and predominant note values. .... . it has to be acknowledged that any such system is only loosely connected to the very patchy and contradictory historical evidence. There is little proof that Bach ever had a fully rationalized system of tempo relations, even if he may have experimented in various ways. Nevertheless, the idea that some such experimentation might be applied to the Mass [in B Minor] can provide the starting point for interpretation if it contributes to a sense of coherence and continuity, something that the work as a whole might seem to demand. It is hardly likely to be very productive as an end in itself."
One point of interest to me in my old article was my finding (late 1990s) that period instruments influenced tempo choice in at least one case: Harpsichordists can't mark a downbeat by playing the note louder, and some tried to convey meter through nuance; to give time for nuancing they tended to play a number of movements in Bach at slower tempi than pianists. This contradicts the stereotype of historical performers who always play Bach much faster than the mainstream. Also, it complicated the view that instruments don't play music, people do. At least in some cases, instruments (in combination with other factors, like a concern with metrical hierarchy) seemed to influence how fast people played, but not in the expected direction. But I'm not sure I had this right, and there have been a lot of recordings since then, so if you find that an interesting idea, you might try testing it out on your own - easy to do!