Despite the instrument indications in the scores, it was common practice for an ensemble to be made up of whatever the parish could scrape together at the time. On any given week the second fiddle player might be down with a kidney stone, and the maestro might give that part to an oboe and fill in the missing oboe part from the organ... whatever worked. Not that different to session playing, is it?

Also, the speed of the piece was not merely a function of how fast the singers/players could go. One fairly standard way to arrange a melody was by "augmentation," meaning you stretched out the notes reeeeaaaal looooonnnnnng and wrote a countermelody to decorate it. Very common in cantatas and in organ preludes.

Bach of course was an undisputed master of countermelodies that often far exceed the beauty of the main melody.

Finally, as an organist, I hope my playing is not mechanical!! (Although there are far too many on all sorts of instruments who play Baroque music too fast and soullessly, IMHO.)

-Glenn