Water flow with plastic and brass pex fittings
Yes, of course it has an effect.  How could it not?  One benefit to using PEX is that you can make very long runs with a single piece of pipe without any joints.
But if you have a concern about a specific run, then the solution is to upsize that run to a size that will flow what you need at that location.
You have not provided any specifics in your question, so the best answer that can be given is also general.