Speed up your 3G/LTE networks using the following tips:
Contact your network provider to check if there is problem with mobile network.
Install your SIM card on another phone to check if there is problem with your phone.
Access Safe Mode by long pressing power key > tapping and holding Power Off > tapping OK . If your phone works well in Safe Mode, there may be some 3rd apps that cause malfunction. Please reboot your phone and check again if it works well.
Backup your data and perform factory data reset.