I don't know if this applies to 2015 Mazda 6, but I bought 2021 GT Reserve recently. The entire infotainment system is woefully slow. I got navigation from Amazon, and adding navigation only made it worse. I've read that the infotainment used to be even slower than it is now, and frankly I am appalled, if that is true.
Since I'm also having tons of problems with my iPod Classic in the USB slot, I may have to "solve" both problems by using iPhone via CarPlay. I don't like it, but at least it behaves as if someone from Mazda tested it, before throwing it over the wall to the customers.
I've read about all of this before I bought the car, and silly me didn't want to believe it could be that bad. My old 2011 BMW M3 had a much better system. sigh
I was initially encouraged by some (old) information on hacking the infotainment system. But those options have been disabled since. Now I'd have to dismantle the dashboard to get in. Not something I want to do on a brand-new car, even though I'd done it before on my previous cars.