slower and slower
At an increasingly slower rate, pace, or velocity. The economy's growth has been getting slower and slower each month, leading some economists to worry that a crash could be on the horizon. The jet flew slower and slower until it eventually began falling from the sky.
slower and slower
at a decreasing rate of speed; slow and then even slower. The car is going slower and slower and will stop soon. The dog's breathing got slower and slower as it went to sleep.