decelerate

verb3

1. •
   To reduce the velocity of something.

   • The S.D.R. [Somerset & Dorset Railway] could not adhere to this schedule, and had to decelerate the service, but the original timings were restored when the M.R. and L.S.W.R. leased the S.D.R. — 1953
2. •
   To reduce the rate of advancement of something, such as a disease.
3. •
   To go slower.