I never understood why anyone cared about single player content in a multiplayer focused game...
That said, I never minded the slowness in Brawl. It's a party game, so I don't expect it to be uber competitive. I just don't like anti-competitive nonsense like tripping or time-only online.
I don't hate it, but tripping was stupid, combos were (and still are) too easy, which leads to repetitive gameplay, and far too much time was spent on the pointless SSE mode.