PROF is KING!
If you're on PROF and want to slow down before the FMS thinks you need to, selecting a slower speed won't work because PROF will take precedence. In this instance you need to change to Speed on Pitch (level change & speed change), or, add drag with the fairly ineffective speed brake, or other drag device (gear/slats) if IAS allows it.
If the aircraft is low on PROF in a level change descent, and you reselect PROF, it will not automatically recapture PROF, it will continue a level change descent to the level in the AAI, unless you select a V/S or FPA that will intercept PROF, then select PROF.