If speed drops over time, there must have been some changes in the environment (e.g. more WiFi cells around your living place or some other devices causing interference such as bluetooth or even microwaves, which are all utilizing the 2.4 GHz frequency band). Be aware that interferences can lead to considerable lower speeds negotiated between the client device and the WiFi AP. Building an optimal WiFi network can be a real challenge. You will have to consider channel selection, WiFi standards, type of antennas, correct placement of the AP etc. TP-Link devices are no exception to this and I would not blame the device for problems arising over time, especially if you already noticed an improvement of TP-Link routers over other brands.

To get best WiFi speed out of any brand of WiFi equipment, try to find the best WiFi channel (the one which isn't overloaded with other WiFi routers) and play with the available standard modes (e.g. 802.11g only in a 2.4 GHz environment or 802.11a only in a 5 GHz environment instead of mixed b/g/n, resp. a/n modes). Also, restrict channel width to 20 Mhz, use WPA2 AES encryption, reduce (!) WiFi signal power, change location of the AP and so on. You also might use one of the free WiFi analyzers for measuring quality or even free programs to produce WiFi heat maps showing the quality of your WiFi cell graphically.

Hope this helps. If not, contact TP-Link support, they are surely glad to help you further.