If your 50mm does not seem right, have it checked at Canon. It does not have true USM, but is plenty fast to focus, faster than most lenses simply because it does not have to move the barrel very far. The focus mechanism can be damaged and slow down the lens focusing. It still appears to be working, but is slow.
The 40mm is a very simple lens. It is slower to focus, but quieter. It is faster to focus on cameras optimized for STM, but there are no FF cameras that are.
As far as sharpness, that's a term that misleads a lot of people, we all use it. I suspect you refer to the overall IQ. For example, a lens image can look very sharp because of high contrast and low chromatic aberrations even though its measured resolution is not that high. http://www.cambridgeincolour.com/tutorials/sharpness.htm
For the price, the 40mm is a super buy, and it produces beautiful images, even at f/2.8