Au contraire, the slowness has to do with the singers. The song was around long before Bach. Lutheran music tends to go slow because the Lutherans let the entire congregation sing, and as any choir leader will tell you, when 200 or more amateurs sing, it's not fair or spiritual to make them go very fast. For another thing, Lutherans just have a tradition that that is how the music of their church ought to sound.

Another reason they did it slow is that the 9/8 part would be left in the dust if they didn't. It taxes the instrument with its big range and uses just about every sharp and flat you can find, fortunately in the coda.

Speaking of little boys, the very obvious 1-3-5-8-5-3-1-3-5-8 bit in the middle of the accompaniment was surely put there to help them come in after a solo recorder passage.