Hi! If you feel your phone is getting slower and slower time by time, try to wipe cache partition. If it still doesnt work, do a factory reset.
Is your phone running on a latest android available for your device? Updating android usually make your phone faster than before.
Or from the begining it was slow? I recommend you to install a custom ROM that can help you speed up your device.
All of the methods i mentioned above can be searched in the internet for better information.
Don't want all the hassle? Consider yourself to buy a new device. Don't forget to do some research and ask on the forums for buying-guide.
P.S: NEVER install a ram booster, speed booster types of application. It does more harm than good. If you have those kind of apps, unninstall them!! It might be the cause of your phone's lag.
Posted with ❤Love❤