The lag time is HORRIBLE and it happens mid-shower, usually when its one of the kids. So we start with a great temperature, it suddenly goes cold and then we wait an age for the heat to come back.
The lag for us would be at the start turning the water on. In winter, that water would have been cold water sitting in the pipe so it needs to come through first as is before the heated water can follow through. If your pipes are insulated or are poly pipes and the run is not from one end of the house to the other, the lag shouldn't be too bad, otherwise there's obviously more cold water that needs to run out first and then the hot water will be used to heat up the cold pipes, so takes longer.
I've been told by our hot water service guy before that if we get temperature fluctuation where water goes hot and cold during shower, it's generally an indication that a service is needed. We tend to have ours done every 2.5 - 3yrs.
Edited: Sorry, just noted that the temperature fluctuation you have is with your gas boosted solar system, not your instantaneous unit. I'm not familiar with solar.