"Bloat" includes the addition of intentional, but frivolous, features. An application can be well-optimized but still slow simply because it's doing too much.
But I think the reason that most modern software performs badly is because of optimization: we're optimizing to reduce production costs over increasing performance.
It's economic in nature. We minimize production costs by using frameworks and other labor-saving tools. The code produced using these tools tends to be poor, but hardware is cheap enough to make up for poorly performing software.
