The GT-i9507 is yet another variant under the Samsung Galaxy S4 banner, taking the number of models of phones called Galaxy S4 up to 24 or 25, (note to self, must change the title of, 23 Galaxy S4 models - know yours?).
The Samsung Galaxy S4 TD-LTE, to give it its correct designation, is similar to the International, quad core, GT-i9505 and is designed specifically for use on Hong Kong and mainland China's networks.
You are on the latest Android 4.3 firmware for this model as KitKat, Android 4.4.2, has not been released for it. I am not familiar with the any carrier added firmware on this phone and so cannot comment on how much added, "bloatware", it has to contend with.
If this lag has occurred since a firmware update, you could read through the Update Problems - Dummies Guide as this contains not only reasons why this may be happening but also some remedies and fixes.
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