As I noted previously, unless you were at or close to exceeding that 64 GB, then adding more RAM would not increase or change render speeds at all - I keep emphasizing that point not to argue, but because I would not want any user going out and buying RAM and expecting a speed increase if their scenes already comfortably fit in RAM, as they won't get a speed increase and will have spent money on nothing. "Spare" RAM that is more than you need adds nothing to render speeds, quality, etc. so in this case, your system must have been using up the full 64GB of RAM (or something else has changed, perhaps something you weren't even aware of).
