The device slowed down after installing security software

Slow file copying, a fan that will not settle, applications that take several seconds to appear. This is the most common complaint about antivirus software, and it has perhaps six causes worth checking in a sensible order.

The short version

For the first day or two after installation, a full initial scan is running and heavy disk activity is expected. If the device is still slow a week later, the usual causes are a scheduled scan running at an unhelpful time, a second security product installed alongside the first, a bundled extra you did not ask for, or hardware that was already close to its limit. Work through them in that order before uninstalling anything.

What happens in the first days

A newly installed security product almost always runs a complete scan of the device. On a machine holding several hundred thousand files, that means reading effectively everything on the disk once, decompressing archives, and building an index so later scans can skip files that have not changed. On a modern solid-state drive this is a background irritation. On an older mechanical hard drive it can take many hours and make the whole machine feel congested while it happens.

Most products will tell you whether an initial scan is in progress, usually on their main screen. If one is, the reasonable thing is to leave the device on, plugged in, and let it finish rather than interrupting it repeatedly — an interrupted scan often starts again from the beginning. Judge the performance afterwards, not during.

The causes, roughly in order of likelihood

A scheduled scan at the wrong time. Many products default to a weekly full scan at a fixed hour. If that hour is one when you are using the machine, the slowdown will seem random and will recur on a weekly rhythm.
A second real-time scanner. Two products inspecting the same file at the same moment is worse than twice the work, because each waits on the other. This is common after a new product is installed on a device that already had one, or when a trial version left components behind.
Real-time scanning of large or busy folders. Video projects, virtual machine disks, game libraries, code repositories and database files all involve writing enormous numbers of changes, and each change invites inspection.
Bundled extras running at startup. A security suite may install a VPN client, a password manager, a browser extension, a backup agent and a system optimiser. Each has its own startup cost, and several may not be components you wanted at all.
A device already short of resources. Eight gigabytes of memory and a mechanical hard drive under a current operating system leave very little headroom. Security software does not create that problem, but it is often what makes it visible.
Something unrelated that coincided. A major operating system update, a failing drive, a full disk, or a browser with sixty tabs and a dozen extensions all produce the same complaint.

Working out which one applies

Look at what is actually consuming the machine. On Windows, open Task Manager with Ctrl+Shift+Esc and sort the Processes list by CPU, then by Disk. On macOS, open Activity Monitor from Applications → Utilities and use the CPU and Disk tabs. You are looking for whether the security product is genuinely at the top, or whether something else is.
Check the scan schedule. Open the product’s settings and find when full scans are set to run. Note the day and hour and compare it against when the slowdown occurs. A weekly pattern is a strong clue.
Check the startup list. On Windows, the Startup apps tab in Task Manager gives each entry a measured startup impact. On macOS, look at Login Items in System Settings. Components of a security suite often appear here individually.
Confirm that only one security product is installed. Go through Installed apps on Windows or the Applications folder on macOS. If you find two, the conflicts page covers removing one properly.
Check free disk space. A drive with almost nothing free will slow everything, and scanning makes heavy use of temporary space. As a rough working figure, aim to keep at least ten to fifteen per cent of the drive free.
Note whether the slowdown is constant or episodic. Constant points at a conflict, a bundled component or the hardware. Episodic points at scheduled scans or particular tasks, such as copying large folders.

Changes usually worth making

Move the scheduled full scan to a time the device is switched on but not in use, which for most households is somewhere in the middle of the day at a weekend rather than overnight when the machine is asleep. Frequency matters less than most people assume: with real-time protection running, a weekly full scan is a reasonable pattern.

Turn off bundled components you do not use. A browser extension, a system optimiser or a backup agent you never configured can usually be disabled without touching the protection itself. If the product will not let you remove a component individually, a custom rather than complete reinstall often will.

Exclusions deserve care rather than enthusiasm. Excluding a specific folder — a virtual machine directory, a video editing scratch folder, a build output directory — from real-time scanning can make a dramatic difference to a specific slow task. Excluding an entire drive, or a general location such as the whole of your user folder, removes protection from exactly the places files usually arrive. Keep exclusions narrow, keep a note of what you excluded, and revisit the list if you later change how you use the machine.

What not to do about a slow machine

Do not leave real-time protection switched off as a performance fix. It is a reasonable thing to pause for a few minutes during a specific task, and a poor thing to forget about for a month.
Do not install a registry cleaner, a driver updater or a system optimiser to compensate. These are a category rather than a product, and the category is closer to the problem than to the solution.
Do not delete files from system folders on the advice of a video or forum post you cannot verify.
Do not grant remote access to anyone who contacts you about the slowdown. Nobody legitimate learns your computer is slow and rings you about it, as the Australian Cyber Security Centre’s guidance at cyber.gov.au reflects.

When it is not the antivirus at all

If Task Manager or Activity Monitor puts something else at the top consistently, the security software is not your problem. Browsers are the usual alternative culprit, particularly with many tabs and several extensions. A drive reporting errors will make everything slow, and both Windows and macOS have built-in disk utilities that will say so. A device that is slow only on the internet, and fine locally, is pointing at the network rather than at the machine.

There is also a plain arithmetic answer that nobody enjoys: a computer that was adequate five years ago is being asked to run software written since. Adding memory or replacing a mechanical hard drive with a solid-state one changes this class of problem more than any setting in any security product will.