Shifting gears problem

My car is taking too long to change/shift gears (it gets very noisy until it finally changes).
If I release the gas pedal when it reaches 25 mph, or 45 mph (etc) it changes smoothly though...
Does anybody know what can be wrong?

if you havent ever changed your fluid, i would do that first. there is a cable that runs from your throttle body down to the tranny, i would inspect aswell