There isn't much point in using a task killer, it doesn't really help, Android is pretty good with handling memory and will lower the priority of background apps if need be, this is especially noticeable on 4.1+ Unless you disable the apps completely through Settings > Apps > Installed then they will just restart using CPU power, which makes the phone slower and uses battery.
What Android version are you using? If you are happy with the risk, you could install a custom ROM that is based on 4.2 or 4.3. That would make the phone much much quicker because Project Butter has definitely helped me
What Android version are you using? If you are happy with the risk, you could install a custom ROM that is based on 4.2 or 4.3. That would make the phone much much quicker because Project Butter has definitely helped me