Can an iMac (Retina 5K, 27-inch, 2020, i5) with 2x4Gb 2666 MHz DDR4 RAM be updated with 2x4Gb 2400 MHz DDR4 RAM (from an iMac Retina 5K, 27-inch, 2017)?

If yes, it is my understanding that the iMac 2020 runs at the level of the older RAM, is that true?

If that is true, would the loss of performance with the mixed setting as described above be large?

What do you think is a better setting (of course 16 Gb 2666 would be the best):

  • only 8 Gb 2666 DDR4 or
  • 16 Gb mixed DDR4 RAM?

The iMac is used for office work and the SWAP is usually around 600 Mb.

Thank you.

  • 1
    My very general experience with mixing RAM speds is that you would be hard pressed to notice the difference. That said you may have some memory intensive applications that would indeed notice the difference. Dec 3, 2020 at 18:24
  • It will always run at the speed of your slowest RAM. You would need to remove the slower RAM to use the speeds of the faster RAM. Dec 3, 2020 at 18:47

1 Answer 1


I would use the mixed ram. When using mixed speeds, your computer will under clock the higher speed cards to match the lower speed cards so they all have the same speed, but since the speeds are so close together (2666 MHz vs. 2400MHz) you wouldn't notice a difference.

An extra 8GB ram is totally worth the 266 MHz speed difference.

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