yavaşla(t)mak, gecik(tir)mek, (hız) azal(t)mak, yavaştan almak, acele etmemek.
He slowed the car down. Economic growth has slowed down dramatically. He needs to slow down a little or he'll get an ulcer.
To slow up (to a stop): Çabucak hızını kesip durdurmak.