It is difficult to say. Some of this is adjustable and some isn't, with many of the details being unique to each pool and thus difficult for us to predict. Most pools do not have this issue, but it is possible that there is nothing you can do about it without replacing some equipment, again depending on details we can't figure out easily and must be discovered by experimenting.
When the solar system turns on the flow rate will go down, how much the flow rate goes down depends on a number of factors, some of which you can adjust and some of which are built into the design of your plumbing system. Solar systems usually have an adjustable diverter valve that controls how much water goes to the panels and how much goes directly to the pool. Adjusting the system so that more water goes directly to the pool will increase the flow rate. However, there is a limit to how much of that you can do without losing most or all of your solar heating. The only way to find out is to experiment with your system.
Other factors can affect flow rate, for example clogged skimmer baskets or a dirty filter. These might cause the symptom to only happen occasionally.