Slow desktop after years of use

The question: when a desktop has gotten slow over the years, how do you tell whether it needs more RAM, an SSD, or just a cleanup? I would like to compare methods, because the usual answer is "buy an SSD" and that is right often enough to be dangerous.

My habit is to measure for ten minutes before I touch a wallet. The machine is telling you where it waits, and most people never ask it.

1. Open Task Manager (or top or iostat on Linux) and use the machine the way you normally do, until it feels slow. Then look, don't guess.
2. Memory: if usage sits near the total and the disk light is busy at the same time, the system is paging. That points to RAM.
3. Disk: if memory has plenty of room but the disk shows near 100 percent active time with long response times, the drive is the bottleneck. On a spinning disk, that is your SSD case.
4. CPU: if one core is pinned with nothing useful running, look for a runaway process or a scan before blaming the hardware.
5. Check the drive's SMART data. Reallocated or pending sectors mean the slowness may be a dying disk, and then the first job is a backup, not a benchmark.

And a few things that cost nothing. Blow the dust out, because a clogged heatsink makes a CPU throttle and looks exactly like age. Check startup items. Look at free space on the system drive, since a nearly full disk slows down on any technology.

I will say the part I am less sure of: where the line is between "add 8 GB" and "this board is too old to bother." I would not put a number on it without testing the actual machine.

What do you look at first? And has anyone been fooled by a cleanup that fixed it for a month and then the real cause came back?

I almost always check ram and processor first I think anything under an i5 (or Core 5 now-a-day) is not really worth the trouble (the generation also matters here). The other thing is I see a lot of people trying to run Windows on 4GB and even 8GB is rough imo.