I don’t know, I only use K-9, but maybe you can ask here instead? FP2 battery saving challenge
EDIT: @Arvil I was seeing K-9 Mail use around 33% of my CPU in the stats. Then I was looking for something else and enabled “Background Sync” in Settings -> Accounts -> three dots. K-9 Mail disappeared from the statistics. Of course I have set it to poll every 30 minutes and use push normally, but it might give you a clue.