So you’ve pressed Shift+F10 again (Run > Run ‘app’) and the emulator is painstakingly slow. Not to worry, because there is Instant Run, a feature that makes the emulator go from a member of the tortoise family to Usain Bolt in a matter of seconds. Plus you get a pretty neat lightning next to you run icon.
First go to
File > Settings > Build, Execution, Deployment > Instant Run
and check Enable Instant Run.
Now when you run your app, then make changes to the code, you can keep the emulator open and so the changes will update a lot faster. You do not need to close the emulator between changes, and the changes made in the code will appear in the emulator almost instantly, thus the name Instant Run.
To get more information about this check out Android Developers Blog post about it.