iPhone 6 - RAM issues, phone running slow, how to fix?

I have an iPhone 6 in great condition –– except that it runs too slow. I'm using iOS 11 though it has been running slow for well over a year and I noticed no difference after updating to iOS 11 (it already was super slow). I've been able to determine that it's probably a RAM issue, because for instance if I have no apps running, but then open Facebook, then YouTube, then then instagram, then Google Maps, then back to Facebook, it restarts Facebook from the beginning (this was not the case during the first year and half of my phone's life). If I open more RAM intensive apps such as Snapchat I am unable to have even two apps running without them restarting when I switch to them.

Anyone know what might be causing this? I downloaded two apps, RAM Monitor and SysCheckLite and it appears that with zero apps open I am using about 80% of my iPhone's RAM.

How can I fix this issue? Is my RAM fried or is there a rogue app (or malware) using up my RAM without me knowing? Would restoring my phone back to factory settings fix this? If so, how do I proceed with that? Also, is does anyone know if there is an app I can download that tells me what apps or system processes are using up my RAM?

Thanks!