So it is necessary to backed up data before hard reset. We can also solve slow speed of mobile y using hard reset method.
Note! All data on mobile memory will be lost like messages, contacts, call history. So take backup important data before hard reset your LG G2 cellphone.
Power off mobile.
Press and hold volume down button and power button together until the LG logo appears on the screen.
When the LG logo is displayed, let go of the power button, keep pressing the volume down key and then press once more the power button.
Once you do that, on your LG G2 the factory reset menu will be displayed.
Press power button in order to agree and wait while the hard reset is being completed.