That doesn’t sound normal. I’m using a phone with much slower processors than yours and Google Maps doesn’t slow the phone down when navigating.
One thing to check is if your phone is overheating during navigating? Putting the phone near the windshield where it’s getting roasted by the sun while running demanding application such as Google Maps navigation might cause your phone’s temperature to spike beyond normal operating range, which cause it to throttle the CPU in order to reduce heat. If your phone is hot, try to put it in front of an AC vent or shaded area in your car.