In my opinion bytafont doesn't hit the battery hard. The first time you run the app it makes a complete backup of your stock fonts. Then when you choose a new font, it replaces the original font. Therefore it basically has the same impact as stock. It loads it all once and then its cached and good to go. At least that's my understanding. I think back in the day it loaded the fonts more frequently but that might also have been FontSwap and not bytafont. Either way its a safe and well used tweak. You shouldn't run into any issues.