A chaconne is supposed to be a dance, right? Bach wrote those note values the way he wrote them for a reason. Did he really want performers to assign any length they felt like assigning them? My gut tells me that he didn’t. I suspect that he probably played his own music in tempo, maybe with some phrasing and ornamentation but still with a clearly recognizable beat. I imagine him gritting his teeth at the rubato that modern performers use. Maybe that’s just me projecting my own preferences, but this sense comes from listening to a lot of Bach and performing some, too.
[AC]: “That’s fundamentally the trouble of trying to apply specific scientific experiments to songs. The whole idea of scientific experiments is to try to control as many things as possible, and sometimes that ends up happening by stripping away some of the things that happen in real life: like lyrics, like meter, and so on. And now you have this song, which has all these extra things that are not part of music cognition experiments — anything you say that the scientific experiments might predict are confounded by the fact that in real life, there are all these other things that weren’t part of the experiment.”