On which device?
If it is your mobile phone or laptop connected to internet then because of too much network traffic in your area this problem arises sometimes but commonly it is the fault of a device.
Storage problems like very small left over space in your phone and too much applications may be installed which may be running in the background consuming a lot of RAM can be sometimes the reason.
Do a try for these possible reasons if problem still persists then i would suggest take backup of your important data and factory Reset your phone or laptop whatever.
And try to keep your phone or laptop as lite as possible.