I have whatsapp, facebook messenger, skype and linphone running on my SailfishX and i did not notice any slowing down. I've yet to experience any serious problem with my dailly use. Polar sync does not work but that is no problem because can sync it when i charge.

Also use other apps like engadged and foreca weather from android without problems.

I do use 4g all the time, what network do you use? I noticed some problems with wifi and android.