I've also got a 2015 Mazda 3 GT and I absolutely love the system. It boots quickly and works really well. I got the CarPlay upgrade done though. Everyone that gets in thinks my car is brand new.
Is that a Mazda installed system? Do you know what version software you have? I know with mine, I had one of the last versions where a Mazda AIO Tweak install was easy. (Later they make it harder.) So maybe there were some improvements made to the official Mazda software in later versions.