Does antivirus slow down your computer?
Less than it used to, but yes, some products noticeably more than others. Independent performance tests show Bitdefender, ESET, and Webroot among the lightest, while heavier suites can slow file copying and app launches on older hardware. On a modern PC the impact of a good product is barely measurable.
The horror stories date from an era when a scan made your PC unusable for an hour. Modern engines scan smarter: they whitelist known-good files, offload analysis to the cloud, and schedule heavy work for idle moments. AV-Comparatives runs a dedicated Performance Test series measuring exactly this, file copying, app installs, launches, and browsing, and the differences between products remain real.
The consistently light performers are ESET (a design priority for decades), Bitdefender (its autopilot defers work intelligently), and Webroot (a cloud-based client of a few megabytes). At the heavier end, full suites that bundle backup, tune-up, and identity layers carry more background processes; Norton and McAfee do more in the background, which older machines feel. Microsoft Defender sits in the middle: usually invisible, though its scheduled scans can be noticeable on budget hardware.
Practical advice: on a reasonably modern computer, do not let performance fears pick your antivirus; protection quality differences matter more than the few percent in benchmarks. On an aging laptop, choose deliberately light (ESET, Webroot, or Defender with scheduled scans set to lunch hours) and skip the all-in-one suites whose extras you will not use anyway.