Every phone needs a care. If you don't care or clean your phone regurarly, it may get slow.
Follow these ways how to speed up your Android's UI under 5 minutes:
Uninstall unused apps
This is one of the most powerful techniques to speed up your phone. You surely remember your phone's speed the first time you bought it - it was mostly because there were no apps.
Remove unnecessary widgets
Yes, widgets slow your phone as well. It's not that bad as with applications but it's also one of the powerful
elements that slow your phone. Get rid of widgets that you don't use especially often updating widgets.
Clear app cache
Clearing the cache of apps may also speed up your Android phone and also free some space. Therefore, it's a good way. However, be careful with cleaning cache of system apps - this might slow your phone.
Apps such as Calender, Drive, Email, Keep and much more keep synchronizating. What does it mean? These apps must run and synchronize with net (therefore take your data also) and uselessly consume the CPU power. You can stop sync in your Google Account information in settings.
Try to install another launcher
User Interfaces such as TouchWiz ROMs may have a complex and challenging interface which may lead to significant deceleration of your Android phone. To solve the problem, simply get a new one.
I personally recommend launchers such as Nova or Apex.
Play with developer options
Settings for developers may also potentially speed up your phone - try to reduce the animation speed to 0.5x
(or disable it completely) or enable GPU rendering. Note these options are hidden - to get access them you have to tap the build number 8 times settings->about phone.