Hi @Caesar,

At first glance, I believe that this may be a side effect of QOS.This may be the router reserving some of the upload speed for new devices that connect. Enabling QOS will allow you to prioritize network traffic towards a specific device.   I would first recommend setting the QOS for your device in the router settings. There will be an option to always have priority or to set a time; and also an option to change the available bandwidth.

Here is an FAQ about how to set up QOS for your devices. 

https://www.tp-link.com/us/support/faq/2797/