This would just affect the performance of the network, in other words: your internet connection.
Connecting directly to your modem or router via a hardwired connection (physical network cable) would eliminate any connection drops associated with wireless traffic. Wireless connectivity issues can occur for a number of reasons, including if you are too far from the router, dead spots in the home, or radio interference.
Yes, for example if you have a microwave between your cell phone and your access point, your connection will suffer until the microwave turns off. Wireless interference / fits and starts / connection drops are all influenced by variables such as other wireless devices, thickness of walls and distance from your access point. A hardwired connection is not susceptible to any of these variables.
What is it that you are using to test your speeds, I am not familiar with T. Typically speed tests are measured in Mpbs. Some of the possible issues could be you need to update your Ethernet drivers need to be update, bad Ethernet cable that would need to be replaced, old network card that supports only 100Mbps.