Iam using Lumia 520 from 2014 ...it's work like a charm 👍 one fine day early morning I noticed that WhatsApp is not working even the application itself is not opening at all.. Few days Later all the apps and even in-built apps are also not working..i thought that its some error or something like that wen I thought after reset every thing will be fine but no use after couple of resets I faced same problem.... I have done hard reset soft reset...and from today evening calendar music player also not working...its like a useless trash mobile .. iam using Windows 8.1 update ..Help me out guys
I Have the same issue.
I think this is a trick used by Microsoft just like apple's does "Apple: Yes, we're slowing down older iPhones" - CNN, 2017.
What worked for me (at least for a few months) was restoring the initial OS using "Windows Device Recovery Tool Installer", you will need to connect your phone to PC via USB and the rest the app will do - Might take a while tho. DON'T FORGET TO BACKUP!