Whenever I connect my Samsung Galaxy S6 to the wifi at home, everything else connected to the internet begins to have connectivity issues
Same issue at my home, I know what's for me: my phone starts uploading photos to my Flickr account, takes a lot of upload band and, of course, download slows down; it is enough to pause Flickr transfer to go back to the usual transfer rate.
Maybe you have any App backup or sending huge amount of data on to the InterNet; you could start to check your data usage statistics on your phone.