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A computer slows down for a handful of predictable reasons: too many programs launching at startup, a nearly full hard drive, outdated software or drivers, malware, or simply not enough RAM for what you're asking it to do. Addressing these one at a time almost always produces noticeable improvement without spending money.
Start with startup programs. On Windows, press Ctrl + Shift + Esc to open Task Manager, click the Startup tab, and disable anything you don't need immediately on boot. On Mac, go to System Settings → General → Login Items and remove the heavy ones. This alone can cut boot time and background RAM usage significantly.
Next, free up disk space. Windows Search for "Disk Cleanup" and run it, or use the Storage Sense setting to auto-delete temporary files. On Mac, Apple menu → About This Mac → Storage → Manage shows what's consuming space. Browsers also cache gigabytes of data over time — clearing the cache (Settings → Clear browsing data) helps.
Keeping Windows Update and macOS Software Update current matters more than most people realize. Updates often include performance patches and driver improvements that directly affect speed. Outdated graphics or storage drivers are a common hidden culprit in sluggish machines.
If the above steps don't help, check for malware using Windows Defender (built-in) or Malwarebytes. If the machine is more than 5–6 years old, adding RAM or replacing a spinning hard drive with an SSD is often the single biggest hardware upgrade possible for under $80.