Hi,
Try starting the phone in [http://howtosafemode.com/375/how-to-turn-on-safe-mode-on-lenovo-k5-note|safe mode] and check if the phone’s performance improves.
If it does then a ''downloaded'' app is the cause of the problem. The trick is to find out which one it is.
You need to uninstall each ''downloaded'' app, one by one, and check the phone’s performance between each uninstall to see if the problem has gone. When it has the last app that you uninstalled will be the culprit.