Depending on the amount of the speed to be reduced. usually with the 737NG if the restriction is about 25 kts less, the vnav will start to break the speed and the vertical speed is drastically reduced at 700 ft or 800 ft before the waypoint altitude untill reaching the new speed. For me, if the vnav does not behave as expected at that moment I do it by myself from MCP (lvl change or V/S first then lvl change)