@aondonatuhwa you are not giving us a lot of information. You need to let us know what you mean by slow. Slow to do what. Anyhow, something like this can in theory be an issue that is related to memory. I would suggest that you backup your phone. After that do a hard reset to set your phone to factory condition. You NEED the original email address and password. The one that was used when you set the phone up.
If you do not have that, do NOT do a factory reset. In that case you could clear your cache and your user data only. Depending on your Android version, check [https://www.youtube.com/watch?v=cdvz2kpqDeY|this video] and see if it works for your phone.