Speeders -
There are 51 countries in Africa. Some are massive, some are tiny.
There are some relatively wealthy larger economies, and some very poor larger economies. There are many relatively wealthy smaller economies, and some very poor smaller economies.
Kenya has 43 million people and a GDP per capita around $2,000.
Tanzania has 45 million people and GDP per capita around $1,600.
Nigeria, which you cite as a negative example,has 170 million people and a GDP per capita of $2,700.
These might seem like very small amounts of money, but in rural, low-cost societies an income of $3,000 may not mean poverty at all.
What is important is that a good dozen countries in Africa are doing very, very well - growing rapidly and establising solid democratic credentials.