Your kinetic energy is always proportional to the square of your speed,
no matter whether you're on a bicycle, a roller coaster, or a baby carriage.
The faster you're moving, the more kinetic energy you have.
Your potential energy doesn't depend on speed at all, only on height.
On a roller coaster, the minimum speeds are usually at the top of a hill
or a loop, so the potential energy would generally correspond to the
slowest speeds. But again, that's not because of the speed. It's because
of the height.