How do I know which applications slows down the phone? How do I know which applications slows down the phone, so that I could get rid of them?
Although you can't find out what apps are 'slowing down' your phone, you can find out which apps are using your battery the most. Generally if apps are using a lot of battery power, they're probably slowing down your phone too.

You can find the list of apps vs battery usage in Settings > Battery.