It will slow down the velocity (feet per second) of the flow but your flow rate (gallons per hour) should remain the same. I can't remember my fluid mechanics stuff from college but I think that's the case.
In my case, I have. 3/4" outlet on my pump the splits up and ends up going through a pair of 1/2" PVC
. Whether I have one plugged or not, the volume of water being moved seems to be the same.