[Q] Restoring performance of m7 topic

Hi there. I have been using my m7 for nearly 2 years now and was going to upgrade to m9 this year. Since m9 dissapointed me really and I have never been a fan of Galaxy series I may stay with my beloved m7 for a bit longer. It is really great phone unfortunately it became laggy and unresponsive some time ago. Not mentioning poor (!!!) battery life... As I was going to upgrade it never actually bothered me enough to think about fixing this. Since now. If I am going to spend at least few more months with this device I would like it to perform as it did before, or close to it.
Updated to Lollipop recently but I cant blame lollipop as performance was bad long ago before update, phone never been rooted, 100% stock, all unwanted apps uninstalled/disabled, only essential apps being used, always Auto sync off for everything, localizations off, I dont use fb, twitter etc only viber sits in the background and Google Now launcher as I got bored with BlinkFeed.

Moving to qeustions... Is this normal for m7 to loose on performance after 2 years usage? Maybe it is just normal for all smartphones after some time and this is just me expecting too much from my "old" ONE? Are there any ways to improve the way my phone works, can involve rooting if necessary so I will finally learn how to do it ;) What are your suggestions friends?

I believe there is many like me and hope this thread will help others as well.

Thanks in advance