Why does my Wi-Fi slow down in the evening?

Almost always congestion, and there are two separate kinds — one inside your home and one outside it.

Outside your home: ISP contention. Broadband is sold on a shared basis. Many households share capacity in the local network and at the ISP's backhaul, and providers size that capacity for typical rather than simultaneous peak use. Between roughly 7pm and 11pm, everyone in your area streams, games and video-calls at once. This is why speeds drop at predictable times and recover overnight, and it affects everyone locally regardless of their equipment.

UK providers must publish peak-time speeds under Ofcom's code, and if actual peak speeds fall well below the advertised minimum you may have grounds to exit a contract.

Inside your home: radio interference. Wi-Fi is a shared radio medium, and your neighbours' networks compete for the same channels.

2.4GHz is heavily congested. It has only three genuinely non-overlapping channels, penetrates walls well — which means your neighbours' signals reach you as effectively as yours reach them — and shares space with Bluetooth, microwaves, baby monitors and cordless phones.

5GHz has far more channels and shorter range, so less neighbour interference, but weaker wall penetration.

Wi-Fi is half-duplex and shared — devices take turns, so total throughput is divided among everything active.

How to tell which you have. Run a speed test wired via Ethernet. If wired speeds are also poor at 8pm and fine at 2am, it is ISP congestion and no equipment change will help. If wired is fine and wireless is not, it is local.

What helps locally: move devices to 5GHz or 6GHz, change channel after scanning what neighbours use, reposition the router centrally and off the floor, use Ethernet for anything stationary, and enable QoS if available.