Your router does two jobs at once. It connects your home to your internet provider, and it broadcasts a local radio network that your devices join. Problems with the first job affect everyone; problems with the second usually affect only some devices or some rooms.
Wi-Fi broadcasts on two main bands. The 2.4 GHz band travels further and passes through walls more easily but carries less data and is crowded, since it is shared with older devices and some appliances. The 5 GHz band is much faster over short distances but fades quickly through solid walls. Many routers publish both under one name and let devices choose.
Each band is divided into channels. If your router and three neighbouring routers all use the same channel, they take turns rather than transmitting at once. Modern routers pick a channel automatically at start-up, which is one reason a restart can visibly improve speed.
When you open a website, your device first asks a name server to translate the address into a number, then sends packets through the router to that destination. If packets are lost along the way they are re-sent, which shows up as delay rather than an error. Understanding this chain is what makes troubleshooting orderly: test the device, then the local network, then the connection out.