Carla, try with following methods:

Power off and then power back on

Powering off the phone for a few seconds will clear and refresh the RAM, returning the device to normal performance.

1. Press and hold the POWER button until the power options appear.
2. Tap Power Off and wait until the phone powers down completely.
3. After a few moments of the phone being powered off, hold the POWER button to power the phone back on.

Check your storage spaceSome apps and processes can run more slowly if there is not enough storage space available or if a faulty storage card is installed.Check your phone's storage in Settings > Storage, and delete or move files if your storage space is too low. If your phone supports a microSD card and there's one installed, try removing or replacing the storage card to see if that helps the issue.

Hi Carla. If the phone is still lagging after factory resetting it, it'll be best to have it checked. It's probably a hardware issue.