Residential internet is oversubscribed, it is a shared medium and ISPs over-sell it by several orders of magnitude (same available bandwidth is typically sold to 20+ people). Peak usage times are usually between 5PM and midnight, and that is when "congestion" occurs, that is typically when it all comes into play. Everyone comes back from work and starts streaming video, browsing, chatting, gaming, etc. The main bandwidth hog being video streaming, especially HD content.
Your modem signal levels play some role in how good your connection is, and it may be more pronounced in peak times. It is sometimes possible to get a better connection with a newer modem, but not very likely. It is also possible to get a different, less congested route if simply your external IP changes. You can sometimes prompt this by changing the MAC address of the NAT router connected to your modem (and rebooting both the modem and the router).. But it requires some time doing traceroutes, testing and noting IP addresses/routes/latency.