Short answer is yes.
Your ISP gives you a set amount of bandwidth, depending on how much you pay them. For this example, say you pay for 25Mbps download speed. If you are on your computer downloading files, you are likely using that entire 25Mbps connection, when another user gets on to try and use the internet they will have a poor experience due to you using most of the bandwidth. It won't cut her off completely, as depending on your router they will try to be fair, however if another user is gaming or streaming videos, they will experience lag and frequent buffering.