Comment by Marazan

Flash was not particularly battery hungry (My go to example when HTML 5 demos started coming out was rebuilding a HTML 5 demo that was using 100% of 1 core into a flash app that used 5%).

The reason it burned CPU cycles is that non-coders could make programs with it and they would produce the world's worst code doing so that "worked". The runtime itself was fine (efficiency wise, not all the other things).