Do you have cable or DSL?
Cable can get congested if there are many people on the same infrastructure at once. Slowdown would occur during peak surfing hours like 6pm to 11pm.
You could have alot of signal noise which could indicate bad lines, a bad tap, a bad splitter, or modem. Your cable provider, as well as you, can see this from viewing your modem. This can be diagnosed by your ISP.
If you are using WiFi, it can be all kinds of things causing this. Poor radio reception and overlapping channels are usually to blame. If you are using WiFi, connect directly to your modem and or router to troubleshoot.
Yes, I’m using WiFi, and paying a friend for use of their Modum which sits nearby ‘pumping stuff out’.