Stop spinning those beachballs
Since I upgraded it, my iMac is slower than before and often shows a spinning beachball, although it has 8GB of memory and a 1TB hard disk with only 269GB used. Should I replace its hard drive with an SSD? An SSD should make your iMac quicker, but that might not be worth the investment required. Before spending any money on it, work out why the beachballs are occurring, and eliminate them. That should make it more nimble again.
The best tools for discovering the causes of beachballs are Activity Monitor and a log browser, such as Console for El Capitan or earlier or Consolation for Sierra or later.
Establish which process(es) are eating up CPU time when the beachball appears, then, from the log, work out what they are struggling with. That could be an outdated app, a conflict, or a process crashing repeatedly.
If Activity Monitor indicates a shortage of free memory, increasing your RAM to the maximum of 16GB could give your Mac a new lease of life. If you intend upgrading to High Sierra and beyond, an SSD would be valuable, since SSDs are well-supported by Apple’s new file system, APFS. But a 1TB SSD would be a substantial cost, and you might be better putting that towards a new iMac with its faster processor and much-improved graphics processor.