I was thinking this was too trivial to post, but really, you guys are talking about furnace filters, so apparently nothing is too trivial.
The kid's laptop (HP Elitebook, i5 3320M, 4GB RAM, 128GB SSD) was performing poorly, not playing Netflix smoothly, lagging a lot on Roblox, basically all simple tasks that it should be able to handle. When I bought it for her before XMAS I identified the 4GB of RAM as needing to be upgraded sometime in the near future, and I was right. Her system was using 100% of system memory. Found 2x4GB sticks of DDR3 1333Mhz SODIMM on Kijiji for $30. They turned out to be Corsair, too. Nice. Swapped them in for the single 4GB stick that was already in there. Problem solved.
I'll list the 4GB stick on Kijiji for $30 and try to make back what I spent on her new RAM.