Tools to see the apps performance

I have an offline application which is using a SQLite database coming from this tutorial SQLite Tutorial, i have problems when installing to the android phone after using the app for a period of time like a minute or using specific function and going back to root page my application seems slower.

For the details of the code refer to my github : Cost Return
Generate apk file : Cost Return apk file

Note : This is my first application hope you understand the code