Why Does WiFi Slow Down When More Devices Connect?

Airtime, not bandwidth, is the scarce resource

Every WiFi frame occupies the channel exclusively while it is transmitted. Devices use a contention protocol (CSMA/CA) to take turns: listen, wait a random backoff, transmit. As the number of active transmitters grows, so does the time spent waiting and colliding rather than sending data.

This is why aggregate throughput on a busy channel can fall well below the throughput of a single client, even though no individual device is saturating its link.

The slow-device problem

A device at the edge of coverage negotiates a low data rate. Sending the same amount of data at 6 Mbps takes roughly a hundred times longer than at 600 Mbps, and during that time nobody else can transmit. One phone in the garden can therefore halve the throughput of a laptop next to the router.

• Legacy 802.11b/g/n clients force protection mechanisms that slow the entire network.
• IoT devices that only support 2.4 GHz crowd the busiest band.
• Devices left connected but idle cost almost nothing; devices transmitting slowly cost a lot.

What actually fixes it

Prioritise changes that reduce contention or move devices to less crowded spectrum.

• Move every capable device to 5 GHz or 6 GHz and keep 2.4 GHz for IoT only.
• Upgrade to WiFi 6 or 7: OFDMA lets the router serve many small clients in a single transmission.
• Add access points wired by Ethernet rather than wireless repeaters, which halve throughput.
• Wire the heaviest consumers — TVs, desktops, consoles — so they leave the air entirely.
• Pick a non-overlapping channel, and use 20 MHz width on 2.4 GHz.

How to confirm contention is the cause

Test the same device twice: once with the household active, once when everything else is off or unplugged. A large gap points at contention rather than the ISP. Testing over Ethernet at the same time isolates the wireless segment definitively.