Wi-Fi shares unlicensed radio space with a surprising amount of household equipment. The 2.4 GHz band in particular is used by microwaves, cordless phones, baby monitors, older wireless doorbells and some Bluetooth traffic.
Interference does not usually break the connection outright. Instead it forces repeated transmissions, so the connection stays up while speed and responsiveness collapse — and often only at certain times of day.
Neighbouring networks are the other half of the story. In dense housing, dozens of routers may sit on overlapping channels, each politely waiting for a gap that never comes.
The practical answer is separation: move devices that can use 5 GHz onto it, keep 2.4 GHz for things that need range, and choose a channel that your neighbours are not sitting on.