I usually try to trackstand at lights while clipped in, but I use clips and straps (mostly so I can wear ultra-hip shoes while I ride, but also cause it's cheap). So anyway, I don't know about unclipping in time cause I just slide my foot out. However, for me, trackstanding is a lot easier while clipped in. Why don't you try this: when you trackstand at lights, try doing so next to something that you could probably grab onto if things get messed up: a lamp post, a parking meter, a street sign, even a car if need be. That way you can build your confidence and have the advantage of being clipped in and more connected to your bike.