Its 1GB ram phone and I can confirm that your ram is getting full and LMK is not able to kill some user installed apps. If you don't believe what i say, you can go to task manager and see the free ram available and apps restarting there.
If you still doubt it, boot into safe mode and your phone will run fast, guaranteed!
I fix these issues and make my own custom roms, kernel tweaks for 1GB phones to make them lag free, that's why i know it [emoji4].