We are wandering a bit off topic, but... It is not a Middle Age thing. For most of history the best guess is that investments have returned around 1 to 2%. It may not even be a stagnation thing. Remember that returns are set by supply and demand. One could have a high growth rate and a high savings rate which would result in a low return on investments. A good example here would be the 19th century Europe. High wealth inequities probably pushed down investment returns here.