There's more than one way to go about testing performance of your game but you usually want to use the Unity's built-in profiler, which has a manual here.
This allows you to see how much of each resources your game is using. You need to dive into the details below and see what's using what, and then optimize by working around the problems or optimizing your code if necessary.
Now, the first way to use the profiler is to just run the game on your computer and look at the results there. This will probably not be very useful by itself since your CPU is probably (and hopefully) much more powerful than your phone's or tablet's. So, you need to limit the CPUs that Unity uses while testing (don't forget to let Unity use all of them when you're done testing). This method is usually much faster than the second one.
The second method would be to use your phone. Either connect your phone to your computer and just use the build and run option Unity provides or send the APK to your phone, install the game and then connect your phone to your computer, then use the profiler to connect to your device. Both will let you test the performance of your game but both are slower to test the performance (in terms of the time it takes from "ready to test the game" to "actually testing the game") than the first method, which allows you to stop the program and then start it again without waiting for a build and an APK installation.