i dont think that huge maps are so much as harder as much as the fact that your empire happens to be bigger so demands more attention and also more enemies\allies you have to deal with and also the larger amounts of resources to capture. So all in all its mor or less more to deal with rather then harder, if you were to get a smaller empire on a huge map then it would seem easyer
also with the speed thing i would assume that has to do with the fact the game is eating up more space and is useing more of your space it would go slower due to that
also with the speed thing i would assume that has to do with the fact the game is eating up more space and is useing more of your space it would go slower due to that