Locknivar Noble Firetube combi not heating water fast enough

We have the Locknivar Noble Firetube combi boiler and it is taking a long time to heat the water (going to the taps). We have well water which is cold. There is also a mixer on the system that adds water from the well to the hot water coming out of the boiler. I would also say the exhaust is pretty loud. If you are standing outside anywhere near the vent it is very noticeable.

Has anyone else noticed the slow water heatup?

Thanks,

Dave