A network cable cannot actually slow down work on the computer, so it is important to test what is slowing down: for example, saving data, in which case it is due to the memory capacity, or streaming content, in which case it is due to full memory. Online, i.e. when connected to the Internet, it may also be because too many people are travelling on the same channel.
Tip: Disconnect the cable again and test what works. And then ask again!