Is slow internet a sign that we have been hacked?
Rarely on its own. Slowness usually has ordinary causes: a backup, an operating system update, a video call, a busy link. It is worth a closer look when it comes with traffic you cannot explain, such as a server uploading large amounts at night, a computer connecting to an unfamiliar destination at regular intervals, or a device that never sent much suddenly sending a lot. Start with your firewall or router: which internal address is using the bandwidth, and where is the traffic going? If the answer is a known job, you are done. A performance monitor shows that a link is busy; NetSenX shows unusual outbound volume per host, judged against that host's own normal, and whether the pattern looks like data leaving.