simplenot I guess FCS does save a lot of battery if several apps are dependant on notification, but what if I only have two (WhatsApp and Signal)?
It depends.
If you are messaging very often (send or receive much – or both), then there will be a very little difference between using FCS or not.
But if you use messaging apps and email apps very rarely, then you will see a big difference. Your battery will last much longer without using FCS.
And if you avoid such evil apps like WA, FB, Insta etc. and use only privacy-friendly messaging apps like Session or Threema, then your battery lasts longer because these apps don't spy on you and will never extract your personal data from your device.
–Signal is not exactly the same, because it needs your phone number and attackers and law enforcement can check every time, where you are and whether your phone is currently unlocked (so that they can surprise you and quickly take it from you in unlocked state if necessary!).
There is an interesting video about this on YouTube (no fake!), but unfortunately I don't have the link to hand. :-(
And remember: the evil Meta synchronises all the data in your address book several times a day (!!!) to ensure that all your contacts are included, regardless of whether they use WhatsApp or not (creation of shadow profiles).
This underhanded criminal behaviour also causes Meta's apps to consume much more energy than other apps.
Reference (there are more on the web, it's one example):
https://itribe.io/how-meta-misuses-user-data-a-deep-dive-into-privacy-violations/