I also just reviewed a massive review article. Just thoroughly reading through it once and noting what I thought the main issues were took several hours. Writing up my comments took a few more. It ended up being rejected for publication. By the end I wasn't quite sure why I participated. I don't work for a university; I work for a small company and it certainly isn't part of my paid work! I guess I view it as a certain professional responsibility (almost like jury duty is), but it doesn't really scale well.