If your device is getting laggy / slow - 99% of the time its down to running out of available RAM. The other 1% is normally down to a badly coded App, hogging all your devices resources.
There's 2 ways you can go about this:
Manually go into Settings/Apps/Running Apps - and then stop each App manually until you cleared enough space in the RAM.
Download a decent App which does it for you. Keep a check on this though as a lot of Apps are a complete waste of time & actually take up more space in your devices RAM than what they actually clear.
Also, any decent App will require you to go into the accessibility settings & enable it there so it can properly stop Apps running.
For that other 1%, you'll need to go into the developer options settings and tick the box next to "Strict mode enabled".This will warn you of any apps that are hogging your devices resources - though use with care (and normally with your device hooked up to a pc & with software on your pc running)
For those feeling ultra brave, whilst you're still in developer options, you can also have a play with the Apps section which is located right at the bottom.
I advise you do some research first before playing with those settings, yet doing it right can bring some pretty amazing results. Again though, if you don't know what you're doing, DON'T mess about with them!!!