Why your internet becomes slow

Every connection has an outside half (router to mast, or router to service provider) and an inside half (router to your devices). Diagnosing the wrong half wastes both money and weeks, so start by establishing which one is at fault.

Connect a laptop to the router by Ethernet cable and run a speed test. Then disconnect the cable, stand next to the router and repeat over Wi-Fi. Finally repeat in the room where the problem is worst. Those three numbers tell you almost everything.

If the wired test is slow, nothing inside your building is at fault and you should read the sections on the mobile link. If wired is fast but Wi-Fi next to the router is slow, the router's Wi-Fi is the issue. If both are fast near the router but slow in the far room, it is a coverage problem.

• Wired slow — mobile link, congestion or plan.
• Wi-Fi slow beside the router — radio configuration, interference or an overloaded router.
• Fine near the router, slow further away — coverage; you need better placement or mesh.
• One device only — that device, not the network.

Mobile capacity at a mast is shared. If your connection is consistently good at nine in the morning and poor from seven in the evening, you are seeing congestion, and that pattern is diagnostic on its own. Log a speed test at the same three times each day for a week and the shape becomes obvious.

Congestion is not something your equipment can fix. Your realistic options are a different operator on the same site, a dual-SIM router that can switch to whichever network is healthier, or an external antenna that lets you hold a cleaner connection to a less loaded cell.

In apartment blocks and dense areas, the 2.4 GHz band is usually crowded with neighbouring networks. Because only three channels genuinely do not overlap, a dozen nearby networks will degrade each other significantly.

Use a Wi-Fi analyser app to see what is around you, then set your 2.4 GHz network to the least crowded of channels 1, 6 and 11 rather than leaving it on an overlapping channel. On 5 GHz there are far more channels and congestion is much less common, which is another reason to keep capable devices on that band.

Non-network interference matters too. Microwave ovens, some cordless phones and poorly shielded electronics all disturb 2.4 GHz. If your slowdown correlates with someone using the kitchen, that is a real and well-documented effect rather than a coincidence.

Count honestly. Phones, laptops, televisions, streaming sticks, cameras, smart plugs, printers and games consoles all hold connections. Older routers begin to struggle with client counts long before their advertised maximum, because the limit is processing capacity rather than an arbitrary number.

A single device can also consume a disproportionate share: a large system update, a cloud backup running in the background, or a security camera uploading continuously. Most router admin pages have a connected-devices or traffic statistics view that will identify the culprit in a couple of minutes.

Quality-of-service settings on better routers let you prioritise video calls over background transfers. On a constrained connection, this often produces a bigger improvement in perceived speed than any bandwidth upgrade.

Heat degrades router performance, and a unit sitting in direct sun or crammed into a closed cabinet will slow down and eventually drop connections. Give it air on all sides and keep it out of sunlight.

A cheap or damaged Ethernet cable will silently negotiate down to 100 Mbps or introduce errors. If a wired device is slow while others are fine, swap the cable before investigating anything else.

Firmware matters too, and so does uptime. Many stability problems are resolved by a firmware update or simply by restarting a router that has been running for months. Restart it before you start buying things.