Aqualisa Aqua Rise Shower - wastes tons of water

We have one of these digital showers, installed about 6 years ago. It works fine but it takes FOREVER to get up to the right temperature (more than 2 minutes) and seems to be getting longer by the day. We have a pressurised Megaflow system with a newish combi boiler. I contacted Aqualisa but they haven't been very helpful yet so wonder if anyone over here has ever experienced this.