What if they a seed site for torrents? Or upload a lot of photos and videos - those have changed usage patters a lot, both cloud backups and YouTube.
High uploads will kill your download speeds as they leave no upstream bandwith for your system's ACKs and complex site browsing data requests.
You should be able to find what is connected to your wifi in your router GUI.
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - AAISP Home::1 80/20. Sync 59997/15142kbps @ 600m. - BQM