Laptop running slow
Find out what's consuming the machine before you change anything. In practice it's nearly always one of four things: startup programs, a full drive, a failing hard disk, or heat.
Look at what's actually running
Windows: Ctrl + Shift + Esc for Task Manager, then sort by CPU, then by Memory, then by Disk.
Mac: open Activity Monitor and do the same across the CPU and Memory tabs.
If one process is pinned near the top, that's your answer and the rest of this list is irrelevant. Search its exact name before killing it.
Cut the startup programs
Software adds itself to startup silently, and after a few years boot time is dominated by things you never open.
Windows: Task Manager → Startup apps → disable everything you don't need immediately at login.
Mac: System Settings → General → Login Items.
Disabling a startup item doesn't uninstall it — you can still launch it normally.
Check free disk space
Drives slow down badly when nearly full, because the system has no room for virtual memory and temporary files.
Keep at least 10–15% free. If you're below that, this may be the entire problem. Empty the trash, clear the Downloads folder, and uninstall what you don't use.
Find out if it's a spinning hard drive
This is the big one on any machine more than a few years old. A mechanical hard disk is the single largest bottleneck in an otherwise fine computer.
Windows: Task Manager → Performance → Disk. It'll say HDD or SSD. If the disk sits at 100% while everything crawls, that's a mechanical drive struggling.
Replacing an HDD with an SSD transforms an old laptop more than any other upgrade, and usually costs less than a new machine.
Rule out overheating
A hot laptop deliberately slows itself down to avoid damage. That's thermal throttling, and it's meant to happen.
Signs: fans loud constantly, hot underside, fine at first then sluggish after twenty minutes. Blow out the vents with compressed air and use it on a hard surface — not a bed or a couch, which block the intakes.
Scan for malware once
Worth one pass, but lower down the list than most advice puts it.
On Windows, run a full scan with the built-in Windows Security — it's genuinely good now and you don't need to buy anything. Don't install three different scanners; they conflict with each other and make the machine slower still.
Consider more memory
If Task Manager or Activity Monitor shows memory consistently near capacity with normal use, adding RAM helps more than any cleanup.
Check first whether your model's memory is replaceable — most recent thin laptops, and all Apple Silicon Macs, have it soldered to the board and cannot be upgraded at all.