>>37454583I think its like this:
Slowpoke has latent psychic power. It wells up inside but its slow to manifest because Slowpoke is adapted to mostly sit around and do nothing to conserve energy.
When it evolves into Slowbro it gains extra psychic power and can tap into it slightly quicker and it can have the balance to walk around on its hind legs, but its still slow because Shellders extra power is in its tail. It still takes time to pump that extra power around.
Slowking is kind of a freak occurrence. The shellder taps right into its psychic power directly and its Brain power is unlocked, letting it talk and act more human-like and wise.But its still pretty passive by nature because that's how they are.