Good question CaptainMongo. I noticed especially on the light A319 climbing at more than 3 to 4000 feet/min the ALT* will engage very early and the speed will often drop quite significantly as it looks like the ALT* takes priority on the speed and assumes you keep the same energy until you are close to the altitude target. I can't find it anymore in the FCOM where it's written but there was a note about this in case you lose one engine during ALT*, the speed might drop significantly as the priority is to the ALT* mode and not the speed mode.
Any more information about it would be cool. =)