If you're a driver then you know that steep curves are taken slowly. (Unless you're going downhill, and in that case it's probably a sign that your brakes have failed...)
The term is often used in common English with the meaning of a difficult initial learning process. Nevertheless, the Oxford English Dictionary, The American Heritage Dictionary of the English Language, and Merriam-Webster’s Collegiate Dictionary define a learning curve as the rate at which skill is acquired, so a steep increase would mean a quick increment of skill.
Arguably, the common English use is due to metaphorical interpretation of the curve as a hill to climb.
Ok, so let me make it graphical for you: If the 'Y' axis of the graph is expenditure of energy or time and the 'X' axis is progress in understanding then a steep curve indicates a lot of expenditure for relatively little progress.
That's the 'informal use', as in, everyday communications such as, but not limited to comments in online communities.
