Adding a pinch of baking soda significantly speeds up the cooking of onion. I've never timed it (and I've never taken onions all the way to caramelization since Indian food doesn't need it), but it's noticeable. It's my go-to trick when cooking a large amount of onions for Indian gravies. As an additional benefit, it also helps cut down the oil requirement, if you're conscious about that sort of thing.
You need to be very careful with how much baking soda you add, really it’s like an 1/8 of a pinch per onion. The smallest amount will brake down a fairly large onion, really quickly.
I’m speaking from experience here, I used what I thought was a small amount on a very large onion and it just destroyed the thing, I had onion mush that wouldn’t even brown correctly.
In my experience even in small amounts the process is faster and the results are inferior. Not enough to ruin the dish unless you overdo it, but enough to not make it worthwhile for me.
Actually some Indian cuisines do need it. I think the best is to add some salt and sugar - that way your onions won't break down but still cook faster.
