Who is your ISP .. where in the world are you.. what make and model is the modem?
This is very typical issue in Australia where I am .. due to Netflix.. it is now called the Netflix Effect. So many people are streaming that all the backhaul is full. Speeds take a dive.
Even disconnecting is possible due to noise in the lines for ADSL or VDSL. The modem will sync at slower speed to compensate. But it will still go slow.
If you set wifi to WPA2 Personal security with a decent password.. say 12 random characters.. you are pretty unlikely to be hacked. I could say impossible a month ago but there is a weakness now called KRACK. However it is a man in the middle type.. a person must live within wifi range of your home, and be able to capture devices when they exchange passwords. So if you live in normal housing estate this is very unlikely. In apartment building with lots of people around you .. the chances increase but not by much. It is also note a weakness in the client not the router unless you are using extend wireless. It is most pronounced in Android OS.. so remove anything that is running Android until you patch it. Of course you should also patch all the other computers and devices ASAP.
Do a simple proof test.
Plug a computer by ethernet to your main modem.. shut off everything else.. and if the modem is a router shut off its wireless.. do a speed test during terrible hours.. if it is slow.. as I suspect it will be.. your wifi is fine. It is ISP issue. Buy internet from a better (more expensive) supplier. Or in the case of Australia.. pray for a government with people who don't live in the previous century. It is inherent in the infrastructure that it grinds to a halt under high demand.