Hi it depends its either your phone storage is full there fore rendering phone system laggy try the following
1.Factory reset by volume + and power
2.Format phones internal storage
3.Root and delete bloat ware
*Try kernel tuner and set low frequency as high as 800 mhz and Maximum to high as 1800 mhz depending on your phone cpu
4.Delete useless apks
If all these fail reload firmaware if still then aproach your nearest cellular technical shop
1.Factory reset by volume + and power
2.Format phones internal storage
3.Root and delete bloat ware
*Try kernel tuner and set low frequency as high as 800 mhz and Maximum to high as 1800 mhz depending on your phone cpu
4.Delete useless apks
If all these fail reload firmaware if still then aproach your nearest cellular technical shop