When a lot people across your network are using the internet at the same time, your online experience tends to slow down somewhat. This can also be a factor if there are multiple people in your home doing high-bandwidth activities at once. Not unlike a busy highway, if all the lanes are jammed with traffic, you have to slow down and get in line — no matter how fast your car can go.
Unfortunately, when it comes to traffic, there's not much you can do about what everyone else is doing. If you have any flexibility, try streaming video or downloading large files during non-peak times. Or, if this isn't possible, you may decide it's time to upgrade your speed or add a second internet line to allow everyone in your home to do what they need to without slowing each other down.