Why does my laptop get slower when it is unplugged?

This complaint is unusual in having a benign explanation most of the time. Every laptop deliberately runs slower on battery, and manufacturers have made those profiles more aggressive over the years because battery life sells machines. Windows exposes it as a power mode slider, macOS as low power mode, and the manufacturer usually adds a third layer of its own. The reason the symptom is worth a guide is the small proportion of cases where it is not the settings — where the battery has degraded to the point that it cannot deliver the current the processor asks for, and the machine throttles itself hard to avoid shutting down. Those two look identical from the outside and have completely different answers.

Step by step

1. Check the obvious setting first.On Windows, Settings, System, Power and battery, and look at the power mode. On a Mac, Settings, Battery, and check whether Low Power Mode is set to apply on battery. Both default to favouring battery life when unplugged, and both can be changed per power source.
2. Look for the manufacturer's own power software.Lenovo, Dell, HP, ASUS and others install a utility with its own performance modes that override the Windows setting. A machine that ignores the Windows power mode is usually being managed by one of these, and the setting you need is in there.
3. Rule out the screen.Auto-brightness dims the screen on battery, and some laptops drop the refresh rate from 120 Hz to 60 Hz to save power. A dim, less fluid screen makes the whole machine feel slower even when it is not. Both are separate settings from the processor profile.
4. Measure rather than judge by feel.Open the same handful of applications plugged in and unplugged and note the difference, or watch the processor speed in Task Manager's performance tab. A modest drop is normal. A processor stuck at a fraction of its rated speed on battery is not.
5. Check the battery's health figure.Windows can produce a battery report from the command line, and macOS shows battery health in settings. A battery down to a low percentage of its original capacity cannot supply peak current, which forces exactly this behaviour. The catalogue's guide on battery health explains how to read the figure.
6. Notice whether it correlates with charge level.Fine at 90 per cent and unusable at 30 per cent is the signature of a worn battery rather than a power profile — the profile does not change as the charge falls, but a tired battery's ability to deliver current does.
7. Test with a different charger before blaming the battery.An underpowered or failing charger can make a machine behave as though it is on battery even while plugged in, and some laptops explicitly detect a non-standard charger and cap performance. Use the manufacturer's charger of the correct wattage.
8. Decide what you actually want.Running Best Performance on battery will roughly halve the runtime on many machines. For most people the right setting is the balanced one, with a deliberate switch to performance for the occasional demanding task, rather than permanently choosing one extreme.

Common mistakes

• Reinstalling Windows to fix it — It is a setting on almost every machine that has it. A reinstall restores the default power profile, which is the very thing causing the symptom, so the fault comes straight back.
• Leaving the machine permanently on maximum performance — It roughly halves battery life and runs the machine hotter, which shortens the battery's life as well. It is a switch to reach for, not a setting to leave.
• Replacing the battery without reading the health figure — The report takes two minutes and tells you whether the battery is the problem. Most of the time it is not, and the fix is free.

If it doesn't work

Questions people ask