I think this is not important when using Instant Run.
David Tiago Conceição

Yes Instant Run is a great improvement, but with API<21 you will get the following trade-offs:

  1. Always deploy full apk on cold swap, see details https://medium.com/google-developers/instant-run-how-does-it-work-294a1633367f#.hu08nle4o
  2. IR will be disabled on multidex project, details https://developer.android.com/studio/run/index.html
  3. IR is still not completely stable and sometimes got stuck on big projects, for these cases it is better to have some other performance hacks

BTW JRebel seems to be more effective on API<21 so it could be another option how to make your build faster, see https://medium.com/@shelajev/looking-at-jrebel-for-android-and-instant-run-how-to-update-code-and-resources-in-android-89b5fc69a98a#.syfpzsydx