Why Is My Mobile Data Slow?

Cause 1 — Weak or noisy signal

Throughput drops sharply as signal quality falls, because the network switches to more robust but slower encoding. Two bars in a busy area can behave worse than one bar in a quiet one, since interference matters as much as raw strength.

Test the same download in two places a few metres apart, ideally one near a window. A large difference points squarely at reception rather than the network.

Cause 2 — Cell congestion

Everyone attached to the same cell shares its capacity. Stations, stadiums, city centres at rush hour and residential areas in the evening all produce slow data for reasons entirely outside your control. If speed recovers late at night in the same spot, congestion is your answer.

Cause 3 — Indoors versus outdoors

Concrete, metal, foil-backed insulation and coated glass attenuate radio signals, and higher frequencies suffer most. This is why a phone showing excellent 5G outside can fall back to slow 4G two rooms inside a building. Step outside and retest before blaming the carrier.

Cause 4 — Network mode and band

5G covers a wide range of frequencies. Low-band 5G reaches far but can be no faster than good 4G, while mid-band delivers the large gains most people associate with 5G. A phone locked to a poor 5G cell sometimes performs worse than it would on 4G, which is why toggling the preferred network type is a legitimate test.

Cause 5 — Your device and its settings

Older modems support fewer bands and less carrier aggregation, which caps peak speed regardless of coverage. Beyond hardware, check the settings that quietly throttle traffic.

• Data saver and low-power modes reduce background and sometimes foreground throughput.
• An active VPN adds encryption overhead and a routing detour.
• Background app refresh, photo sync and app updates compete for the same radio.
• A stale connection sometimes clears with airplane mode on and off, which forces reattachment to the network.
• Very high device temperature causes deliberate throttling of the modem.

Cause 6 — Plan limits

Many plans reduce speed after a data allowance is used, and some prioritise other traffic during congestion. If speed dropped abruptly on a specific date and has stayed low everywhere since, check your allowance and plan terms before troubleshooting the radio.

A quick diagnostic sequence

Test outdoors and indoors, at a busy hour and a quiet one, with the VPN off, and once after toggling airplane mode. Four measurements are usually enough to tell reception, congestion, configuration and plan limits apart.