I think immortality could rather slow down the evolution/progress of a society. Think about it: New ideas mostly come from the younger generation while the old generation is rather opposed to it. That problem is usually solved when the older generations die.
For example, nowadays younger people all use computers, smartphones etc., while some older people refuse to even buy a computer, and would propably continue to do so for a long time, if they were immortal.
But more importantly, some centuries ago, only a few people fought for democracy, while most others were opposed to it. That seems to be one of the reasons the Weimar Republic failed.