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.

This is not normal at all. Reflash your phone, or at the very least ensure it isn't thermal throttling. Even a poxy mediatek chipset doesn't do this.

Is the phone maybe overheating and throttling, due to being exposed to direct sunlight on the dash? Wireless charging can have the same effect

Google Maps is just trash.

Something wrong there.

I've a Samsung A71, SnapDragon 730, certainly much less capable than your phone, and I can switch apps while using google maps without any performance hit. I usually drive/ride with Waze though.

Does it happen only when it's in your car, or anytime you use google maps? Do you have android auto?

Do you have many map details switched on? (3d buildings, traffic, air quality, etc)

Have you tried it right after rebooting the phone?