First, let see list of reasons why my android phone was so slow all of a sudden;

1) ROM

2) RAM

3) Background Apps

4) Wifi connection (this aid more of background app update compared to a data connection)

5) Phone processor speed

I experienced the same and did some basic thing:

1) Bought a new Android tablet (yours could be a phone) but what matter is that I ensured the RAM of my new phone was twice the previous one. From 2GB to 4GB RAM

2) It also twice in term of ROM, from 8GB to 64GB storage space

3) It has a higher processor speed

4) Installed background apps hibernation app which really helps

5) Follow the more detailed helpful guide from JoyofAndroid for more tips on solving your slow phone here - Slow Android Phone – How To Speed It Up, What Causes It, How to Fix It 

unnecessary tasks are running from the phone.