Some are bigger, like certain US cities (Chicago and even Pittsburgh, which isn't near as big as London.) Reading, PA (my nearest city) is very quick, but you'll definitely need to randomize some buildings. Don't ever randomize too many. I don't use vanilla jobs so I always make work 0% but total home, leisure and shopping is no more than 2%. Usually real city size is related to how long it takes/how likely to crash a less capable setup. You need to randomize almost always anyway as almost no cities have all types of business - especially strip club, brothel and beauty.