Why these two parts
A computer feels slow for a handful of reasons, and two of them are cheap to fix. Not enough memory forces the system to shuffle data to storage; slow storage makes everything that touches the disk crawl.
Replacing a mechanical hard drive with an SSD is still the single most dramatic upgrade available to an older laptop. Adding memory is the second.
Check what is possible before you buy
Upgradability varies by model, and increasingly the answer is no:
- Memory may be socketed (SODIMM), and replaceable, or soldered to the board and fixed.
- Storage may be an M.2 slot, a 2.5-inch bay, or soldered flash.
Look up the exact model before buying. Teardown guides and service manuals will tell you what is socketed, what tools are needed, and how hard the machine is to open.
What each upgrade changes
More memory helps most when you keep many things open at once: browser tabs, a chat app, a spreadsheet, a video call. It does not make a single program run faster.
A faster drive changes how quickly everything starts and how responsive the machine feels, which is usually what people mean when they say a computer is slow.
For buyers of refurbished hardware
An upgraded machine is worth more than the parts cost, which is why good refurbishers upgrade before listing rather than after.
If you are buying, the practical move is to compare like for like: a listing with 16GB and an SSD against one with 8GB and a hard drive is not a comparison of the same computer.