The loose flowing movements of tai chi are very deliberately slow because gravity is maintained when you move slowly. You can align the body to work with gravity and improve your balance. Leaning of any kind will destabilise the body.
It is fairly easy to move quickly. Try lifting a leg in slow motion... the leg feels very heavy and hard to move. When your body submits to gravity, it can feel as if you are lifting weights every time you practice tai chi. Stronger bones and muscles are developed as a consequence.