An aging mechanical hard drive
Needs hardware
This is the big one, and it's the reason I won't sell a cleanup to every caller. If your computer still has a spinning mechanical drive, that drive is the bottleneck. Windows waits on it constantly, so everything feels slow: boot, opening programs, switching between windows. Spinning drives also get slower as they age and sectors start to fail. Clearing out files and startup programs makes a dent, but the machine is still waiting on the same slow part. Replacing it with a solid-state drive is the fix, and on an older machine it typically does more for everyday speed than anything else you could spend the money on.
Not enough RAM for how you use it
Needs hardware
If you keep a lot of browser tabs open and the computer slows to a crawl, it has likely run out of memory and started using the drive as overflow, which is far slower. 4GB is not enough for current Windows. 8GB is workable, 16GB is comfortable. Adding RAM is a hardware fix, and whether it's possible depends on the machine, since some laptops have it soldered to the board.
Startup clutter and background software
A cleanup helps
Years of installed programs that all want to launch at boot and keep running. Manufacturer utilities, old printer software, three different updaters, a browser carrying a dozen extensions. This is genuinely worth clearing out, and on a machine that otherwise has decent hardware it can make a real difference.
Malware or an adware infection
A cleanup helps
Something is using your processor or network in the background. This often comes with popups, a browser homepage you didn't choose, or search results going somewhere unexpected. If that's what's happening, the virus and malware cleanup is the job you want rather than a tune-up.
Overheating and dust
Cleaning helps
When a computer runs too hot it deliberately slows itself down to avoid damage. Clogged vents, a failing fan, or old dried-out thermal paste all cause it. The tell is a machine that's fine for ten minutes and then gets sluggish, often with the fan running hard.
A nearly full drive
Often free to fix
Windows needs free space to work in. Under about 10% free and things get noticeably worse. Sometimes this is the whole problem and clearing space is all it takes, which is the kind of thing I'd rather just tell you about on the phone.