If your upload channel is full it does not matter if you have ADSL, WiFi, or full duplex fiber. The actual technology only makes it worse or slightly better, but your connection will start to feel very slow.
The reason is that almost all kinds of communication over the Internet involve use of both channels. You send requests and get data. Also, TCP (the protocol used to carry almost everything on the Internet today, including HTTP) uses “acknowledgements”, packets that serve to signal that you’re receiving data and the sender can continue sending more data to you.
In other words: if you are sending a immense amount of data, your upload channel will get full. If you try to download something big at the same time, you’ll need some bandwidth going out to signal the sender that you are receiving the data. If your upload is so full to the point that this “ack” message gets ‘lost’, the sender stops sending – and your Internet feels slow.