All of these answers are wrong!

Wrong, wrong, WRONG!

The best way to speed up an old laptop isn’t to buy more RAM, or even to replace a hard disk drive with an SSD.

No, the single best “upgrade” you can do to an old computer is really an important bit of maintenance, and doesn’t involve new hardware at all. Just a $5 tube of grease:

This is called thermal paste. Its job is to fill the microscopic gaps between your CPU and the heatsink that cools it off, so that heat gets out of the CPU as fast as it can. When the thermal paste isn’t working right, your laptop has to kill performance to keep from overheating.

The problem is, manufacturers almost always use low-quality paste to save money. That doesn’t really hurt things when the computer is new, but over time that crappy thermal paste is guaranteed to dry out and stop working. At the same time, older computers have to run harder as programs get more demanding, which puts more “load” on the thermal paste.

This is part of why old laptops get mysteriously hotter and slower with age, even compared to new computers that should be the same speed.

So, pretty much the single best upgrade you can do – both in terms of performance-per-dollar and overall system stability – is to just pull off your heatsink and replace the old thermal paste with some better stuff.

If the old paste was fine, you haven’t wasted much money and still get a small benefit – but if it was a problem, the difference will be absolute night and day.

The reason I’d call this hands-down the “best” upgrade is, unlike a lot of other computer trouble, bad thermal paste doesn’t have a lot of workarounds. Either you replace it, or your computer is permanently slow and hot.

This is what the thermal paste on my 6-year-old laptop looked like when I took its heatsink off – less of a paste and more of a powder:

With $2 of new paste, an hour of disassembly, and a little bit of cleaning, this computer runs faster, quieter, and 15 degrees cooler at idle.

Can’t beat that.

Edit for all the SSD comments:

Just because someone suggests one upgrade doesn’t mean something else is worthless. Upgrading to an SSD is still a great choice, and “best upgrade” doesn’t mean “only thing you should do”

But you have to know why you’re doing things, and what problems they solve. New thermal paste is important because there’s no other way to fix that problem.

An SSD is good for faster disk reads – like when starting up a computer and launching programs, but in a healthy computer disk reads shouldn’t happen often. However, since your computer can also use the disk as extra memory in an emergency (called paging), sometimes everything winds up being faster. That’s a sign that what you really needed was more RAM, and bridging that gap with an SSD won’t work as well.

Doing a lot of paging on an SSD can make it fail a lot faster, so I’d generally recommend actually diagnosing what the problem is before just whacking one in.