Forget RAM, forget a faster CPU, forget a beefier graphics card. If you are still running a Mac with a spinning, physical hard drive, the best upgrade you can possibly make is to drop a solid-state flash drive into the machine. The immediate effect on perceived performance is stunning: it’s the difference between seeing a spinning beach ball every hour and not seeing one for months at a time.
Unfortunately, for a long time, what has kept most people from making this update to their older Macs has been price. SSDs are more expensive than physical HDDs. That’s still true, by the way, but it’s less so now than it ever has been, making this a perfect time to finally take the plunge.