Ars Technica:

Unlike phones and tablets, which can still post big performance gains from year to year, desktops age more slowly and gracefully. A typical replacement cycle in many businesses and schools is three or four years, and, as long as they don’t break, you can easily keep using them for years after that.

Apple has lent us its top-end 5K iMac to test, but instead of just sticking to year-over-year performance comparisons, we’ll be going all the way back to 2012 to compare it against some of the older iMacs that it might end up replacing (we’ve also included the 2011 iMac in a few cases, though it can’t run all of the benchmarks that newer iMacs can). A lot has changed in five years, but how much faster have things really gotten? We’ll also get into the handful of technological updates Apple has made since the last new iMacs came out in late 2015.

There are some really interesting and geeky stats in this story but the bottom line is, if your three years or older iMac feels “sluggish”, you’ll notice significant speed gains on the newest iMacs.