This question already has an answer here:
I found a lot of info about how to make my smartphone faster but I couldn't find the reason for smartphones lowliness.
- I found that you should move apps to SD Card (if you have one)
- Don't use a lot of widgets, try not to use too much home screens.
- Remove apps that you don't use. Turn off animation.
And more stuff like this.
But what makes the phone become slow with time?