Fast lenses are often remarkably different in character at the fast end. Even older f/2 lenses can be pretty distinctive, my LTM Canon 50/1.9 being one example - wide open, it's got all sorts of corner distortion, it's soft all around, and it glows, while the 50/1.5 from the same era is a little soft and low contrast but overall far better, even though it's ~2/3rds of a stop faster. The difference is down to completely different optical formulas, the 50/1.9 being a double-gauss Planar and the 50/1.5 a Sonnar. How lens designers get to their target maximum aperture is often more important than the number they stick on the front of the lens.
FWIW, I love the 50/1.9 as a night street photography lens. With HP5+ pushed to 1600 it's got a really classic look, and you don't really notice the imperfections.